Low Temperature Electrolytes for Solid Oxide Cells Having High Ionic Conductivity

ABSTRACT

Methods for forming a metal oxide electrolyte improve ionic conductivity. Some of those methods involve applying a first metal compound to a substrate, converting that metal compound to a metal oxide, applying a different metal compound to the metal oxide, and converting the different metal compound to form a second metal oxide. That substrate may be in nanobar form that conforms to an orientation imparted by a magnetic field or an electric field applied before or during the converting. Electrolytes so formed can be used in solid oxide fuel cells, electrolyzers, and sensors, among other applications.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims benefit ofpriority of U.S. Non-Provisional patent application Ser. No. 15/597,126,filed on May 16, 2017, now U.S. Pat. No. 10,344,389, which is acontinuation of and claims benefit of priority of U.S. Non-Provisionalpatent application Ser. No. 14/981,097, filed on Dec. 28, 2015, nowabandoned, which is a continuation and claims benefit of priority ofU.S. Non-Provisional patent application Ser. No. 13/578,195, filed onAug. 9, 2012 and having a § 371 date of Jan. 28, 2013, which representsthe National Stage under 35 U.S.C. § 371 of International PatentApplication No. PCT/US2011/024242, filed internationally on Feb. 9,2011, which in turn claims benefit of priority under PCT Article 8 and35 U.S.C. § 119(e) of U.S. Provisional Application No. 61/303,003, filedon Feb. 10, 2010. The foregoing Ser. Nos. 15/597,126, 14/981,097,13/578,195, PCT/US2011/024242, and 61/303,003 applications areincorporated herein by reference in their entireties.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support awarded by theDepartment of Energy and administered by Oak Ridge NationalLaboratory/UT Battelle. The government has certain rights in theinvention.

FIELD OF THE INVENTION

The present invention relates to electrical energy systems such as fuelcells, electrolyzer cells, and sensors, and, in particular, to solidoxide fuel cells, solid oxide electrolyzer cells, solid oxide sensors,and components of any of the foregoing.

BACKGROUND OF THE INVENTION

Solid oxide fuel cells, otherwise known as ceramic fuel cells, presentan environmentally friendly alternative to mainstream electrical energyproduction processes involving the combustion of fossil fuels. Solidoxide fuel cells enable the catalytic conversion of chemical energystored in hydrogen into electrical energy without the concomitantrelease of greenhouse gases. The generation of electrical current by asolid oxide fuel cell using a hydrogen fuel results in the production ofwater as opposed to the production carbon dioxide, nitrous oxides,and/or sulfur dioxides associated with the combustion of fossil fuels.

In addition to hydrogen, solid oxide fuel cells are operable to functionon a wide variety of fuel sources. Fuel sources in addition to hydrogeninclude hydrocarbons such as methane, natural gas, and diesel fuel.Hydrocarbon fuel sources are reformed into hydrogen for use with solidoxide fuel cells. Hydrocarbon reforming can be administered prior toentry into the fuel electrode or can be administered at the fuelelectrode of a solid oxide fuel cell. The ability to function on a widevariety of fuels distinguishes solid oxide fuel cells from other fuelcells which lack the ability to operate on various fuels. Furthermore,the ability of solid oxide fuel cells to administer hydrocarbonfeedstock reformation frees such fuel cells from the limitationsassociated with hydrogen production and distribution.

Currently, solid oxide fuel cells operate at high temperatures rangingfrom about 800° C. to 1000° C. As a result of high operatingtemperatures, solid oxide fuel cells require the use of exotic materialswhich can withstand such operating temperatures. The need for exoticmaterials greatly increases the costs of solid oxide fuel cells, makingtheir use in certain applications cost-prohibitive. High operatingtemperatures exacerbate stresses caused by differences in coefficientsof thermal expansion between components of a solid oxide fuel cell. Ifthe operating temperature could be lowered, numerous advantages could berealized. First, less expensive materials and production methods couldbe employed. Second, the lower operating temperature would allow greateruse of the technology. Third, energy needed to heat and operate the fuelcell would be lower, increasing the overall energy efficiency.Significantly, the high operating temperature is required because ofpoor low temperature ion conductivity.

Proton exchange membrane (“PEM”) fuel cells enjoy operationaltemperatures in the range 50-220° C. Typically relying on specialpolymer membranes to provide the electrolyte, PEM cells transmit protonsacross the electrolyte, rather than oxygen ions as in solid oxide fuelcells. However, high proton conductivity requires precise control ofhydration in the electrolyte. If the electrolyte becomes too dry, protonconductivity and cell voltage drop. If the electrolyte becomes too wet,the cell becomes flooded. Electro-osmotic drag complicates hydrationcontrol: protons migrating across the electrolyte “drag” water moleculesalong, potentially causing dramatic differences in hydration across theelectrolyte that inhibit cell operation. Accordingly, it would beadvantageous to obtain the low operating temperatures of the PEM fuelcell without the need to maintain strict control over electrolytehydration.

In certain circumstances, a solid oxide fuel cell can operate “inreverse” to electrolyze water into hydrogen gas and oxygen gas byinputting electrical energy. In other circumstances, a solid oxideelectrolyzer cell can be designed primarily for use as a hydrolyzer,generating hydrogen and oxygen for later use. In still othercircumstances, an electrolyzer cell can be used for other purposes, suchas extraction of metal from ore and electroplating. In conventionalelectrolyzers, electrical energy is lost in the electrolysis reactiondriving the diffusion of ions through the electrolyte and across thedistance between the electrodes. Also, the ability to conductelectrolysis at higher temperatures would improve the efficiency of theelectrolysis. However, at higher temperatures, electrolyzers facesimilar thermal stresses and cracking caused by differences incoefficients of thermal expansion between components of the solid oxideelectrolyzer cell. Accordingly, better matching of coefficients ofthermal expansion and lower operating temperatures are desired forelectrolyzer cells.

A lambda sensor is a device typically placed in the exhaust stream of aninternal combustion engine to measure the concentration of oxygen. Thatmeasurement allows regulation of the richness or leanness of thefuel/air mixture flowing into the engine. If the fuel/air streamcontains too much oxygen, the quantity A is greater than 1, and themixture is too lean. If the fuel/air stream contains too little oxygen,then λ<1 and the mixture is too rich. λ equals 1, the ideal situation,when the mixture contains a stoichiometrically equivalent concentrationof oxygen and hydrocarbon to allow for complete combustion. A lambdasensor positioned in the exhaust stream detects the amount of oxygen inthe combustion products, thereby providing feedback regarding richnessor leanness. Lambda sensors and other sensors rely on the diffusion ofoxygen anions (O²⁻) and other ions through barrier materials in wayssimilar to the manner in which oxygen anions diffuse through a solidelectrolyte of a solid oxide fuel cell. Moreover, given the highoperating temperature of lambda sensors and similar devices, sensorsface thermal stresses, cracking, and delamination issues similar tothose facing fuel cells and electrolyzers. Accordingly, embodiments ofthe present invention provide for improved sensor technology byaddressing ionic conductivity and mismatching of coefficients of thermalexpansion, among other reasons.

It has recently been reported that adjacent atomically flat layers ofstrontium titanate (STO) with yttria-stabilized zirconia (YSZ) producean interface that has a dramatically higher ionic conductivity foroxygen anions. J. Garcia-Barriocanal et al., “Colossal IonicConductivity at Interfaces of Epitaxial ZrO₂:Y₂O₃/SrTiO₃Heterostructures,” 321 SCIENCE 676 (2008). Those authors concluded thatgrowing thin epitaxial layers of YSZ on epitaxial STO caused the YSZ toconform under strain to the crystal structure of the STO, therebycreating voids in the YSZ crystal structure at the interface between thetwo materials. Those voids allowed an increase of oxygen ionicconductivity of approximately eight orders of magnitude relative to bulkYSZ at 500 K (227° C.).

In view of the foregoing problems and disadvantages associated with thehigh operating temperatures of solid oxide cells, it would be desirableto provide solid oxide cells that can demonstrate lower operatingtemperatures. In addition, providing solid oxide cells and componentsthat better tolerate higher temperatures would be advantageous.Moreover, the efficiency losses due to the thickness of electrolytesmake thinner electrolytes desirable. Furthermore, it is also desirableto construct metal oxide electrolytes having dramatically higher ionicconductivities. Large-scale production of metal oxide electrolytes wouldbe facilitated if higher ionic conductivities could be achieved withoutrequiring epitaxial growth of electrolyte materials.

SUMMARY

Applicants have unexpectedly discovered methods for fabricating metaloxide electrolytes for use in solid oxide cells that do not requirepainstaking epitaxial growth of electrolyte materials, in someembodiments of the present invention. In other embodiments, unexpectedlyhigh ionic conductivities can be observed. In still other embodiments,unexpectedly high ionic conductivities can be observed at relatively lowtemperatures. Without wishing to be bound by theory, certain embodimentsexhibit enhanced ionic conduction by providing domain boundaries (forexample, crystal grain boundaries) disposed in a direction parallel tothe desired ionic conduction.

As used herein, “solid oxide cell” means any electrochemical cell thatcontains a metal oxide electrolyte, and refers to, for example, solidoxide fuel cells, solid oxide electrolyzer cells, cells that can operateas a fuel cell and an electrolyzer cell, and solid oxide sensors.

“Metal oxide electrolyte” indicates a material, useful as an electrolytein a solid oxide cell, that contains a metal oxide. The metal oxideelectrolyte can contain one or more metal oxides dispersed in anysuitable manner. For example, two metal oxides can be mixed together inthe manner of ZrO₂:Y₂O₃, or SrTiO₃. For another example, two metaloxides can be present in discrete domains having an abrupt interfacebetween them. In yet another example, two metal oxides can form adiffuse interface between them. Still further examples provide more thantwo metal oxides present in a metal oxide electrolyte, such as, forexample, ZrO₂:Y₂O₃/SrTiO₃. The metal oxide electrolyte optionallyfurther contains a material other than a metal oxide. Examples include,but are not limited to, metals, semiconductors, insulators (other thanmetal oxides), carbides, nitrides, phosphides, sulphides, and polymers,and combinations thereof. In the context of this disclosure, siliconepolymers are polymers, while silica is a metal oxide. When used in thisdocument, the meaning of “material” includes metal oxides unlessotherwise indicated.

Accordingly, some embodiments of the present invention relate to methodsof enhancing ionic conductivity in a metal oxide electrolyte comprisinga first material and a metal oxide comprising:

applying a metal compound to the first material; andconverting at least some of the metal compound to form the metal oxide;wherein the first material and the metal oxide have an ionicconductivity greater than the bulk ionic conductivity of the firstmaterial and of the metal oxide.

Other embodiments provide a metal oxide electrolyte comprising:

a first material and a metal oxide, wherein the metal oxide is formed byapplying a metal compound to the first material; andconverting at least some of the metal compound to form the metal oxide,wherein the first material and the metal oxide have an ionicconductivity greater than the bulk ionic conductivity of the firstmaterial and of the metal oxide.

Still other embodiments provide methods for forming a metal oxideelectrolyte, comprising:

applying a metal compound to a first material in powder form; andconverting at least some of the metal compound to form a metal oxide,thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first material and of the metaloxide.

Additional embodiments provide methods for forming a metal oxideelectrolyte, comprising:

applying a first metal compound to a substrate;converting at least some of the first metal compound to form a firstmetal oxide on the substrate;applying a second metal compound to the substrate comprising the firstmetal oxide; andconverting at least some of the second metal compound to form a secondmetal oxide on the substrate comprising the first metal oxide,thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first metal oxide and of thesecond metal oxide.

Still other embodiments provide methods for forming a metal oxideelectrolyte, comprising:

applying a metal compound to a first material in nanobar form; andconverting at least some of the metal compound to form a metal oxide,thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first material and of the metaloxide. Nanobars, in the present invention, comprise single wallednanotubes, multiwalled nanotubes, nanorods, and combinations thereof. Incertain embodiments, the nanobars comprise a material susceptible toorient in an electric or magnetic field, such as, for example,ferroelectric materials, ferromagnetic materials, and paramagneticmaterials, alone or in combination. In one embodiment, a nanobar has aperovskite crystal structure. In another embodiment, the nanobar furthercomprises a derivative that imparts a dipole moment to the nanobar. Inyet another embodiment, a nanobar comprises a segnetoelectric material,such as, for example, those disclosed in International ApplicationPublication No. WO/2005/019324, which is incorporated herein byreference in its entirety. A segnetoelectric material exhibits apolarization even in the absence of an external electric field. Suchspontaneous polarization is caused by the crystal structure of thematerial, and a given material may have segnetoelectric andnonsegnetoelectric crystal phases. Barium titanate, for example,exhibits segnetoelectric behavior. Piezoelectric materials may alsoexhibit segnetoelectric behavior. In further embodiments, one or moreorienting forces can be applied, such as, for example, brushing, spincoating, a magnetic field, an electric field, or a combination thereof,to cause the nanobars to assume an orientation in the electrolyte. Theorienting force can be applied before and/or during the converting.

Certain other embodiments of the present invention provide methods forforming a metal oxide electrolyte comprising:

applying a metal compound to a thin sheet; andconverting at least some of the metal compound to form a metal oxide onthe thin sheet, thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the thin sheet and of the metaloxide. In some embodiments, a thin sheet comprises mica.

Yet further embodiments provide a solid oxide cell, comprising:

an inner tubular electrode having an outer surface;an outer electrode; anda metal oxide electrolyte adapted to provide ionic conductivity betweenthe inner tubular electrode and the outer electrode;

-   wherein the metal oxide electrolyte comprises a plurality of thin    sheets oriented substantially perpendicular to the outer surface of    the inner tubular electrode, and a metal oxide contacting the thin    sheets.

Certain embodiments of the present invention provide enhanced ionicconductivity through the metal oxide electrolyte, thereby allowing alower operating temperature. By lowering the operating temperature of asolid oxide cell, less exotic and easier-to-fabricate materials can beutilized in the construction of the cell leading to lower productioncosts. Thus, some embodiments of the present invention provide solidoxide cells and components thereof employing simpler, less-expensivematerials than the current state of the art. For example, if theoperating temperature of a solid oxide cell can be lowered, then metalscan be used for many different components such as electrodes andinterconnects. At these lower operating temperatures, metals have moredesirable mechanical properties, such as higher strength, than ceramics.In addition, this higher strength can allow metal components also tohave a higher degree of porosity. Current ceramic electrode materialsallow for porosity levels in the range of 30% to 40%. Incorporatinghigher porosity levels in ceramic materials renders them toostructurally weak to support cell construction. However, through the useof certain metals or metal carbides, the porosity of an electrode can beprovided in the higher range of 40% to 80% and yet retain sufficientmechanical strength for cell construction. Some embodiments of thepresent invention provide an electrode having a porosity ranging fromabout 40% to about 80%.

Lower production costs in addition to lower operating temperaturesprovide the opportunity for solid oxide cells to find application in awider variety of fields. Additionally, lower operating temperaturesreduce degradative processes such as those associated with variances incoefficients of thermal expansion between dissimilar components of thecell. Accordingly, some embodiments provide means and methods forreducing a degradation process in a solid oxide cell.

Still other embodiments produce a desirable surface catalytic effect.For example, by using the process of some embodiments of the presentinvention, thin films of metal oxides and pure metals (or other metalcompounds) can be formed on the exposed pore surfaces of electrodes toproduce more chemically active sites at triple phase boundaries whereeither fuel-gas (as in the case of the anode electrode) or gaseousoxygen (as in the case of the cathode electrode) come into contact withthe solid (yet porous) electrodes in a fuel cell.

Other embodiments provide methods of making solid oxide cells andcomponents thereof. Certain embodiments provide methods of making solidoxide cells and components thereof applying temperatures dramaticallybelow those of current methods. Current methods of making solid oxidefuel cells involve the sintering of ceramic and/or metal powders. Highsintering temperatures during fabrication of various components, such asthe electrolyte, can compound problems associated with variances incoefficients of thermal expansion.

These and other embodiments are described in greater detail in thedescription which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures are not necessarily to scale, and should not be construed aslimiting. Some details may be exaggerated to aid comprehension.

FIG. 1 is a micrograph at approximately two million×magnification thatillustrates a thin film of yttria-stabilized zirconia (“YSZ”: a materialthat can be used to produce ceramic electrolytes in solid oxide cells)with an interlayer (106) between the pure YSZ thin film (102) and thepure stainless steel (grade 304) of the substrate (104). The mixedYSZ-oxide & substrate interlayer (106) appears between the lower steelsubstrate layer (104) and the upper YSZ-oxide layer (102).

FIG. 2 illustrates a solid oxide fuel cell according to one embodimentof the present invention.

FIG. 3 partially illustrates a solid oxide cell according to oneembodiment of the present invention. A first material comprising apowder 350 and a metal oxide 360 form a metal oxide electrolyte 380between two electrodes 310, 320. When operated as a fuel cell, oxygenanions diffuse, among other places, through interfaces between thepowder 350 and the metal oxide 360.

FIG. 4 partially illustrates a solid oxide cell according to oneembodiment of the present invention. A first metal oxide 450 and asecond metal oxide 460, disposed in interpenetrating domains of metaloxides, form a metal oxide electrolyte between two electrodes 410, 420.When operated as a fuel cell, oxygen anions diffuse, among other places,through interfaces between the first metal oxide 450 and the secondmetal oxide 460.

FIG. 5 partially illustrates a solid oxide cell according to oneembodiment of the present invention. A nanobar 540 and a metal oxide560, disposed so that the nanobars 540 orient substantiallyperpendicularly to a first planar electrode 510, form a metal oxideelectrolyte between two electrodes 510, 520. When operated as a fuelcell, oxygen anions diffuse, among other places, through interfacesbetween the nanobar 540 and the metal oxide 460.

FIG. 6 comprises FIG. 6A and FIG. 6B. FIG. 6A partially depicts anotherembodiment of the present invention, a plurality of thin sheets 650comprising metal oxide 660 between the thin sheets 650. FIG. 6B depictsa view of cut “A” from FIG. 6A.

FIG. 7 comprises FIG. 7A and FIG. 7B. FIG. 7A partially depicts anotherembodiment of the present invention, a plurality of thin sheets 750 inannular form arranged substantially concentrically and substantiallyparallel. FIG. 7B partially depicts a side cut-away view of a tubularsolid oxide cell according to another embodiment of the presentinvention. A plurality of thin sheets such as those depicted in FIG. 7Aform a metal oxide electrolyte 780 between two tubularconcentrically-arranged electrodes 710, 720.

FIG. 8 partially depicts a solid oxide cell according to a furtherembodiment of the present invention, optionally operable to test a metaloxide electrolyte 880 for enhanced ionic conductivity. A cathode 810 andan anode 820 sandwich a metal oxide electrolyte 880 to test performancewith external circuitry 870.

DETAILED DESCRIPTION

The present invention provides solid oxide cells, components thereof,and methods of making and using the same.

Electrolytes

Some embodiments of the present invention include electrolytes andmethods for making electrolytes having enhanced ionic conductivity.Ionic conductivity is the rate at which one or more ions move through asubstance.

Ionic conductivity generally depends upon temperature in most solidelectrolytes, and is usually faster at higher temperature. In somecases, poor ionic conductivity at room temperature prevents economicaluse of certain fuel cell technologies. Accordingly, enhancing ionicconductivity can provide either more efficient solid oxide celloperation at a given temperature, or operation at a lower temperaturethat is thereby rendered efficient enough to be economically feasible.

Ionic conductivity can relate to any ionic conductivity, such as, forexample, the conductivity of monoatomic, diatomic, and multiatomic ions;monovalent, divalent, trivalent, tetravalent, and other multivalentions; cations; anions; solvated and partially-solvated ions, andcombinations thereof. In some embodiments, ionic conductivity concernsthe conductivity of O²⁻. In other embodiments, ionic conductivityconcerns the conductivity of O²⁻, H⁺, H₃O⁺, OH⁻, NH₄ ⁺, Li⁺, Na⁺, K⁺,Mg⁺, Ca⁺, F⁻, Cl⁻, Br⁻, I₃ ⁻, I⁻, and combinations thereof. Ionicconductivity is often reported in units of 1/(ohms cm) or S/cm, where 1S=1 A/V. In context of the present invention, ionic conductivity isenhanced if, in reference to a literature or experimental value of bulkionic conductivity of the most-ionic conductive material in the metaloxide electrolyte, the ionic conductivity has increased by astatistically significant amount. In some embodiments, the ionicconductivity has increased at least one order of magnitude, from aboutone order of magnitude to about two orders of magnitude, from about twoorders of magnitude to about three orders of magnitude, from about threeorders of magnitude to about four orders of magnitude, from about fourorders of magnitude to about five orders of magnitude, from about fiveorders of magnitude to about six orders of magnitude, from about sixorders of magnitude to about seven orders of magnitude, from about sevenorders of magnitude to about eight orders of magnitude, from about eightorders of magnitude to about nine orders of magnitude, from about nineorders of magnitude to about ten orders of magnitude, or greater thanabout ten orders of magnitude.

Certain embodiments of the present invention relate to methods ofenhancing ionic conductivity in a metal oxide electrolyte comprising afirst material and a metal oxide comprising:

applying a metal compound to the first material; andconverting at least some of the metal compound to form the metal oxide;wherein the first material and the metal oxide have an ionicconductivity greater than the bulk ionic conductivity of the firstmaterial and of the metal oxide. In those embodiments, the firstmaterial may provide a substrate for the formation of the metal oxide,or the first material and the metal compound are depositedsimultaneously or sequentially on a substrate for the converting. Thus,the first material may be in any suitable physical form, from thinsheets or films to powders to nanobars, in some embodiments. When thefirst material is present in a powdered form, the first material cancomprise particles having an average size or diameter of less than about1 cm, or less than about 0.5 cm, in some embodiments. In otherembodiments, the first material in powdered form has an average size ordiameter ranging from about 2 nm to about 0.5 cm, or from about 2 nm toabout 10 nm, or from about 10 nm to about 50 nm, from about 50 nm toabout 100 nm, from about 100 nm to about 250 nm, from about 250 nm toabout 500 nm, from about 500 nm to about 1 micron, from about 1 micronto about 5 microns, from about 5 microns to about 50 microns, from about50 microns to about 100 microns, from about 100 microns to about 250microns, from about 250 microns to about 500 microns, from about 500microns to about 1 mm, from about 1 mm to about 5 mm. The powder cancomprise particles of any suitable shape, including but not limited tospheres, pyramids, cubes, polygons, irregular polygons, cylinders,nanobars, discs, flakes, irregularly-shaped solids, and combinationsthereof. For shapes having a high aspect ratio, the average size refersto the largest dimension of the shape, such as the length of a cylinderor the diameter of a disk. Some embodiments provide a first material inpowder form comprising mica, and a metal oxide comprisingyttria-stabilized zirconia, gadolinium-doped ceria, alumina, or acombination thereof.

The first material, in certain embodiments, can comprise, among otherthings, crystalline material, nanocrystalline material, metal oxides,nanobars, mica flakes, thin sheets, and combinations thereof.Crystalline material includes single crystals and material that has beenformed epitaxially, such as by atomic layer deposition. In furtherembodiments, the first material is chosen from strontium titanate,titania, alumina, zirconia, yttria-stabilized zirconia, alumina-dopedyttria-stabilized zirconia, iron-doped zirconia, magnesia, ceria,samarium-doped ceria, gadolinium-doped ceria, and combinations thereof.Additional embodiments provide the first material being chosen fromalumina, titania, zirconia, yttria-stabilized zirconia, alumina-dopedyttria-stabilized zirconia, iron-doped zirconia, magnesia, ceria,samarium-doped ceria, gadolinium-doped ceria, and combinations thereof.

In some embodiments, detection of a given material need not requirecrystallographic analysis. For example, alumina-doped yttria-stabilizedzirconia refers to oxide material comprising aluminum, yttrium,zirconium, and oxygen. Accordingly, detection of constituent elementssignifies the indicated material. Elemental detection methods are widelyknown, and include, but are not limited to, flame emission spectroscopy,flame atomic absorption spectroscopy, electrothermal atomic absorptionspectroscopy, inductively coupled plasma spectroscopy, direct-currentplasma spectroscopy, atomic fluorescence spectroscopy, andlaser-assisted flame ionization spectroscopy.

Mica appears as flakes, chunks, thin sheets, or a combination thereof,in certain embodiments of the present invention. “Mica,” as used in thepresent disclosure, refers to a family of readily-cleavable materials,synthetic or naturally-occurring, also known as phyllosilicates.Biotite, muscovite, phlogopite, lepidolite, margarite, and glauconite,and combinations thereof, are types of mica that can be used.

Certain embodiments provide the first material in the form of a thinsheet. In some of those embodiments, the first material comprises atleast one thin sheet. Thin sheets of material, such as, for example,mica, metal oxides, conductors, semiconductors, and insulators, can beused. Some embodiments employ thin sheets of MgO, BaTiO₃, NaCl, KCl,alone or in combination. Also, thin sheets are chosen from crystallinematerial such as slices of single crystal and epitaxial films grown on asubstrate and optionally removed from that substrate. Other materialsthat can be used provide a thin sheet that can withstand thetemperatures of processing and operation. In certain cases, thatmaterial is not electrically conductive, to avoid shorting out the solidoxide cell. In other cases, metal oxide or other electrical insulator isinterposed between the conductive flat sheet and at least one electrode,to avoid shorting out the cell. For example, the electrodes can compriseone or more alike or different metal oxide coatings formed by applyingat least one metal compound to the electrode, and converting at leastsome of the at least one metal compound to at least one metal oxide.

In some embodiments, a thin sheet has a thickness ranging from about 1micron to about 10 microns, from about 10 microns to about 50 microns,from about 50 microns to about 100 microns, from about 100 microns toabout 200 microns, from about 200 microns to about 500 microns. In otherembodiments, a thin sheet has a thickness of less than about 1 micron,or greater than about 500 microns. Optionally, one or more epoxies areused to fill in any defects or to seal a thin sheet.

When the first material comprises a thin sheet, in some embodiments, thefirst material is present in the solid oxide cell in a plurality ofalike or different thin sheets. In certain embodiments, those thinsheets are oriented substantially parallel to each other, andsubstantially perpendicular to one or more electrodes. Thus, in theoperation of the cell, ion diffusion through the metal oxide electrolyteoccurs in a direction roughly parallel to the plane of the thin sheet,rather than through (or perpendicular to) the thin sheet. Thin sheets ofceramics, minerals, metal oxides, and combinations thereof can be usedin metal oxide electrolytes in certain embodiments of the presentinvention.

Some embodiments of the present invention provide at least one metaloxide chosen from strontium titanate, titania, alumina, zirconia,yttria-stabilized zirconia, alumina-doped yttria-stabilized zirconia,iron-doped zirconia, magnesia, ceria, samarium-doped ceria,gadolinium-doped ceria, and combinations thereof. In other embodiments,the metal oxide is chosen from alumina, titania, zirconia,yttria-stabilized zirconia, alumina-doped yttria-stabilized zirconia,iron-doped zirconia, magnesia, ceria, samarium-doped ceria,gadolinium-doped ceria, and combinations thereof.

In still further embodiments, the metal oxide electrolyte comprises afirst material comprising strontium titanate, and a metal oxidecomprising yttria-stabilized zirconia. In other embodiments, the firstmaterial comprises magnesia, and the metal oxide comprisesyttria-stabilized zirconia. Additional embodiments have a first materialcomprising titania, and a metal oxide comprising yttria-stabilizedzirconia. Yet other embodiments provide a first material comprisingstrontium titanate, and a metal oxide comprising iron-doped zirconia.Certain embodiments include a first material comprising samarium-dopedceria, and a metal oxide comprising ceria.

Some additional embodiments provide yttria-stabilized zirconiacomprising from about 10 mol % to about 20 mol % yttria, from about 12mol % to about 18 mol % yttria, or from about 14 mol % to about 16 mol %yttria.

Applying one or more metal compounds to one or more materials can occuraccording to any suitable method. Dipping, spraying, brushing, mixing,spin coating, and combinations thereof, among other methods, can beused. Then the metal compound is converted to form at least one metaloxide in the presence of the material, and optionally in the presence ofa substrate. In certain embodiments, the metal compound is fullyconverted to a metal oxide. A metal compound composition comprises ametal-containing compound that can be at least partially converted to ametal oxide. In some embodiments, the metal compound compositioncomprises a metal carboxylate, a metal alkoxide, a metal β-diketonate,or a combination thereof.

A metal carboxylate comprises the metal salt of a carboxylic acid, e.g.,a metal atom and a carboxylate moiety. In some embodiments of thepresent invention, a metal salt of a carboxylic acid comprises atransition metal salt. In other embodiments, a metal salt of acarboxylic acid comprises a rare earth metal salt. In a furtherembodiment, metal carboxylate compositions comprise a plurality of metalsalts of carboxylic acids. In one embodiment, a plurality of metal saltscomprises a rare earth metal salt of a carboxylic acid and a transitionmetal salt of a carboxylic acid.

Metal carboxylates can be produced by a variety of methods known to oneskilled in the art. Non-limiting examples of methods for producing themetal carboxylate are shown in the following reaction schemes:

nRCOOH+Me→(RCOO)_(n)Me^(n)+0.5nH₂ (for alkaline earth metals, alkalimetals, and thallium)

nRCOOH+Me^(n+)(OH)_(n)→(RCOO)_(n)Me^(n+) +nH₂O (for practically allmetals having a solid hydroxide)

nRCOOH+Me^(n+)(CO₃)_(0.5n)→(RCOO)_(n)Me^(n+)+0.5nH₂O+0.5nCO₂ (foralkaline earth metals, alkali metals and thallium)

nRCOOH+Me^(n+)(X)_(n/m)→(RCOO)_(n)Me^(n+) n/mH_(m)X (liquid extraction,usable for practically all metals having solid salts)

In the foregoing reaction schemes, X is an anion having a negativecharge m, such as, e.g., halide anion, sulfate anion, carbonate anion,phosphate anion, among others; n is a positive integer; and Merepresents a metal atom. R in the foregoing reaction schemes can bechosen from a wide variety of radicals.

Suitable carboxylic acids for use in making metal carboxylates include,for example:

Monocarboxylic Acids:

Monocarboxylic acids where R is hydrogen or unbranched hydrocarbonradical, such as, for example, HCOOH—formic, CH₃COOH—acetic,CH₃CH₂COOH—propionic, CH₃CH₂CH₂COOH (C₄H₈O₂)—butyric, C₅H₁₀O₂—valeric,C₆H₁₂O₂—caproic, C₇H₁₄—enanthic; further: caprylic, pelargonic,undecanoic, dodecanoic, tridecylic, myristic, pentadecylic, palmitic,margaric, stearic, and nonadecylic acids;

Monocarboxylic acids where R is a branched hydrocarbon radical, such as,for example, (CH₃)₂CHCOOH—isobutyric, (CH₃)₂CHCH₂COOH—3-methylbutanoic,(CH₃)₃CCOOH—trimethylacetic, including VERSATIC 10 (trade name) which isa mixture of synthetic, saturated carboxylic acid isomers, derived froma highly-branched C₁₀ structure;

Monocarboxylic acids in which R is a branched or unbranched hydrocarbonradical containing one or more double bonds, such as, for example,CH₂═CHCOOH—acrylic, CH₃CH═CHCOOH—crotonic,CH₃(CH₂)₇CH═CH(CH₂)₇COOH—oleic, CH₃CH═CHCH═CHCOOH—hexa-2,4-dienoic,(CH₃)₂C═CHCH₂CH₂C(CH₃)═CHCOOH—3,7-dimethylocta-2,6-dienoic,CH₃(CH₂)₄CH═CHCH₂CH═CH(CH₂)₇COOH—linoleic, further: angelic, tiglic, andelaidic acids;

Monocarboxylic acids in which R is a branched or unbranched hydrocarbonradical containing one or more triple bonds, such as, for example,CH≡CCOOH—propiolic, CH₃C≡CCOOH—tetrolic, CH₃(CH₂)₄C≡CCOOH—oct-2-ynoic,and stearolic acids;

Monocarboxylic acids in which R is a branched or unbranched hydrocarbonradical containing one or more double bonds and one or more triplebonds;

Monocarboxylic acids in which R is a branched or unbranched hydrocarbonradical containing one or more double bonds and one or more triple bondsand one or more aryl groups;

Monohydroxymonocarboxylic acids in which R is a branched or unbranchedhydrocarbon radical that contains one hydroxyl substituent, such as, forexample, HOCH₂COOH—glycolic, CH₃CHOHCOOH—lactic, C₆H₅CHOHCOOH—amygdalic,and 2-hydroxybutyric acids;

Dihydroxymonocarboxylic acids in which R is a branched or unbranchedhydrocarbon radical that contains two hydroxyl substituents, such as,for example, (HO)₂CHCOOH—2,2-dihydroxyacetic acid;

Dioxycarboxylic acids, in which R is a branched or unbranchedhydrocarbon radical that contains two oxygen atoms each bonded to twoadjacent carbon atoms, such as, for example, C₆H₃(OH)₂COOH—dihydroxybenzoic, C₆H₂(CH₃)(OH)₂COOH—orsellinic; further: caffeic, and pipericacids;

Aldehyde-carboxylic acids in which R is a branched or unbranchedhydrocarbon radical that contains one aldehyde group, such as, forexample, CHOCOOH—glyoxalic acid;

Keto-carboxylic acids in which R is a branched or unbranched hydrocarbonradical that contains one ketone group, such as, for example,CH₃COCOOH—pyruvic, CH₃COCH₂COOH—acetoacetic, andCH₃COCH₂CH₂COOH—levulinic acids;

Monoaromatic carboxylic acids, in which R is a branched or unbranchedhydrocarbon radical that contains one aryl substituent, such as, forexample, C₆H₅COOH—benzoic, C₆H₅CH₂COOH—phenylacetic,C₆H₅CH(CH₃)COOH—2-phenylpropanoic, C₆H₅CH═CHCOOH—3-phenylacrylic, andC₆H₅C—CCOOH—3-phenyl-propiolic acids;

Multicarboxylic Acids:

Saturated dicarboxylic acids, in which R is a branched or unbranchedsaturated hydrocarbon radical that contains one carboxylic acid group,such as, for example, HOOC—COOH—oxalic, HOOC—CH₂—COOH—malonic,HOOC—(CH₂)₂—COOH—succinic, HOOC—(CH₂)₃—COOH—glutaric,HOOC—(CH₂)₄—COOH—adipic; further: pimelic, suberic, azelaic, and sebacicacids;

Unsaturated dicarboxylic acids, in which R is a branched or unbranchedhydrocarbon radical that contains one carboxylic acid group and acarbon-carbon multiple bond, such as, for example,HOOC—CH═CH—COOH—fumaric; further: maleic, citraconic, mesaconic, anditaconic acids;

Polybasic aromatic carboxylic acids, in which R is a branched orunbranched hydrocarbon radical that contains a aryl group and acarboxylic acid group, such as, for example, C₆H₄(COOH)₂—phthalic(isophthalic, terephthalic), and C₆H₃(COOH)₃—benzyl-tri-carboxylicacids;

Polybasic saturated carboxylic acids, in which R is a branched orunbranched hydrocarbon radical that contains a carboxylic acid group,such as, for example, ethylene diamine N,N′-diacetic acid, and ethylenediamine tetraacetic acid (EDTA);

Polybasic Oxyacids:

Polybasic oxyacids, in which R is a branched or unbranched hydrocarbonradical containing a hydroxyl substituent and a carboxylic acid group,such as, for example, HOOC—CHOH—COOH—tartronic,HOOC—CHOH—CH₂—COOH—malic, HOOC—C(OH)═CH—COOH—oxaloacetic,HOOC—CHOH—CHOH—COOH—tartaric, and HOOC—CH₂—C(OH) COOH—CH₂COOH—citricacids.

A metal compound composition, in some embodiments of the presentinvention, comprises a solution of carboxylic acid salts of one or moremetals (“metal carboxylate”). A liquid metal carboxylate composition cancomprise a single metal, to form a single metal carboxylate, or amixture of metals, to form a corresponding mixture of metalcarboxylates. In addition, a liquid metal carboxylate composition cancontain different carboxylate moieties. In some embodiments, a liquidmetal carboxylate composition contains a mixture of metals, as thesecompositions form mixed oxides having various properties.

Solvent used in the production of liquid metal carboxylate compositions,in some embodiments, comprise an excess of the liquid carboxylic acidwhich was used to form the metal carboxylate salt. In other embodiments,a solvent comprises another carboxylic acid, or a solution of acarboxylic acid in another solvent, including, but not limited to,organic solvents such as benzene, toluene, chloroform, dichloromethane,or combinations thereof.

Carboxylic acids suitable for use generating liquid metal carboxylatecompositions, in some embodiments, are those which: (1) can form a metalcarboxylate, where the metal carboxylate is soluble in excess acid oranother solvent; and (2) can be vaporized in a temperature range thatoverlaps with the oxide conversion temperature range.

In some embodiments, a carboxylic acid has a formula R—COOH, where R isalkyl, alkenyl, alkynyl or aryl.

In some embodiments, the monocarboxylic acid comprises one or morecarboxylic acids having the formula I below:

R^(∘)—C(R″)(R′)—COOH  (I)

wherein:R^(∘) is selected from H or C₁ to C₂₄ alkyl groups; andR′ and R″ are each independently selected from H and C₁ to C₂₄ alkylgroups; wherein the alkyl groups of R^(∘), R′, and R″ are optionally andindependently substituted with one or more substituents, which are alikeor different, chosen from hydroxy, alkoxy, amino, and aryl radicals, andhalogen atoms.

The term alkyl, as used herein, refers to a saturated straight,branched, or cyclic hydrocarbon, or a combination thereof, including C₁to C₂₄, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl,n-pentyl, cyclopentyl, isopentyl, neopentyl, n-hexyl, isohexyl,cyclohexyl, 3-methylpentyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl,heptyl, octyl, nonyl, and decyl.

The term alkoxy, as used herein, refers to a saturated straight,branched, or cyclic hydrocarbon, or a combination thereof, including C₁to C₂₄, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl,n-pentyl, cyclopentyl, isopentyl, neopentyl, n-hexyl, isohexyl,cyclohexyl, 3-methylpentyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl,heptyl, octyl, nonyl, and decyl, in which the hydrocarbon contains asingle-bonded oxygen atom that can bond to or is bonded to another atomor molecule.

The terms alkenyl and alkynyl, as used herein, refer to a straight,branched, or cyclic hydrocarbon, including C₁ to C₂₄, with a double ortriple bond, respectively.

Alkyl, alkenyl, alkoxy, and alkynyl radicals are unsubstituted orsubstituted with one or more alike or different substituentsindependently chosen from halogen atoms, hydroxy, alkoxy, amino, aryl,and heteroaryl radicals.

Moreover, the term aryl or aromatic, as used herein, refers to amonocyclic or bicyclic hydrocarbon ring molecule having conjugateddouble bonds about the ring. In some embodiments, the ring molecule has5- to 12-members, but is not limited thereto. The ring may beunsubstituted or substituted having one or more alike or differentindependently-chosen substituents, wherein the substituents are chosenfrom alkyl, alkenyl, alkynyl, alkoxy, hydroxyl, and amino radicals, andhalogen atoms. Aryl includes, for example, unsubstituted or substitutedphenyl and unsubstituted or substituted naphthyl.

The term heteroaryl as used herein refers to a monocyclic or bicyclicaromatic hydrocarbon ring molecule having a heteroatom chosen from O, N,P, and S as a member of the ring, and the ring is unsubstituted orsubstituted with one or more alike or different substituentsindependently chosen from alkyl, alkenyl, alkynyl, hydroxyl, alkoxy,amino, alkylamino, dialkylamino, thiol, alkylthio, ═0, ═NH, ═PH, ═S, andhalogen atoms. In some embodiments, the ring molecule has 5- to12-members, but is not limited thereto.

The alpha branched carboxylic acids, in some embodiments, have anaverage molecular weight ranging from about 130 to 420 g/mol or fromabout 220 to 270 g/mol. The carboxylic acid may also be a mixture oftertiary and quaternary carboxylic acids of Formula I. VIK acids can beused as well. See U.S. Pat. No. 5,952,769, at col. 6, II. 12-51, whichpatent is incorporated herein by reference in its entirety.

In some embodiments, one or more metal carboxylates can be synthesizedby contacting at least one metal halide with at least one carboxylicacid in the substantial absence of water. In other embodiments, thecontacting occurs in the substantial absence of a carboxylic anhydride,yet in specific embodiments at least one carboxylic anhydride ispresent. In still other embodiments, the contacting occurs in thesubstantial absence of a catalyst; however, particular embodimentsprovide at least one catalyst. For example, silicon tetrachloride,aluminum trichloride, titanium tetrachloride, titanium tetrabromide, ora combination of two or more thereof can be mixed into 2-ethylhexanoicacid, glacial acetic acid, or another carboxylic acid or a combinationthereof in the substantial absence of water with stirring to produce thecorresponding metal carboxylate or combination thereof. Carboxylicanhydrides and/or catalysts can be excluded, or are optionally present.In some embodiments, the carboxylic acid is present in excess. In otherembodiments, the carboxylic acid is present in a stoichiometric ratio tothe at least one metal halide. Certain embodiments provide the at leastone carboxylic acid in a stoichiometric ratio with the at least onemetal halide of about 1:1, about 2:1, about 3:1, or about 4:1. Thecontacting of the at least one metal halide with at least one carboxylicacid can occur under any suitable conditions. For example, thecontacting optionally can be accompanied by heating, partial vacuum, andthe like.

Either a single carboxylic acid or a mixture of carboxylic acids can beused to form the liquid metal carboxylate. In some embodiments, amixture of carboxylic acids contains 2-ethylhexanoic acid wherein R^(∘)is H, R″ is C₂H₅ and R′ is C₄H₉, in the formula (I) above. The use of amixture of carboxylates can provide several advantages. In one aspect,the mixture has a broader evaporation temperature range, making it morelikely that the evaporation temperature of the acid mixture will overlapthe metal carboxylate decomposition temperature, allowing the formationof a metal oxide coating. Moreover, the possibility of using a mixtureof carboxylates avoids the need and expense of purifying an individualcarboxylic acid.

Other metal compounds can be used to form metal oxides in accordancewith the present invention. Such metal compounds can be used alone or incombination, or in combination with one or more metal carboxylates.Metal compounds other than carboxylates and those mentioned elsewhereinclude metal alkoxides and metal β-diketonates.

Metal alkoxides suitable for use in the present invention include ametal atom and at least one alkoxide radical —OR² bonded to the metalatom. Such metal alkoxides include those of formula II:

M(OR²)_(z)  (II)

in which M is a metal atom of valence z+;z is a positive integer, such as, for example, 1, 2, 3, 4, 5, 6, 7, and8;R² can be alike or different and are independently chosen fromunsubstituted and substituted alkyl, unsubstituted and substitutedalkenyl, unsubstituted and substituted alkynyl, unsubstituted andsubstituted heteroaryl, andunsubstituted and substituted aryl radicals,wherein substituted alkyl, alkenyl, alkynyl, heteroaryl, and arylradicals are substituted with one or more alike or differentsubstituents independently chosen from halogen, hydroxy, alkoxy, amino,heteroaryl, and aryl radicals. In some embodiments, z is chosen from 2,3, and 4.

Metal alkoxides are available from Alfa-Aesar and Gelest, Inc., ofMorrisville, Pa. Lanthanoid alkoxides such as those of Ce, Nd, Eu, Dy,and Er are sold by Kojundo Chemical Co., Saitama, Japan, as well asalkoxides of AI, Zr, and Hf, among others. See, e.g.,http://www.kojundo.co.jp/English/Guide/material/lanthagen.html. Examplesof metal alkoxides useful in embodiments of the present inventioninclude methoxides, ethoxides, propoxides, isopropoxides, and butoxidesand isomers thereof. The alkoxide substituents on a give metal atom arethe same or different. Thus, for example, metal dimethoxide diethoxide,metal methoxide diisopropoxide t-butoxide, and similar metal alkoxidescan be used. Suitable alkoxide substituents also may be chosen from:

1. Aliphatic series alcohols from methyl to dodecyl including branchedand isostructured.2. Aromatic series alcohols: benzyl alcohol—C₆H₅CH₂OH; phenyl-ethylalcohol—C₈H₁₀O; phenyl-propyl alcohol—C₉H₁₂O, and so on.Metal alkoxides useful in the present invention can be made according tomany suitable methods. One method includes converting the metal halideto the metal alkoxide in the presence of the alcohol and itscorresponding base. For example:

MX_(z) +zHOR²→M(OR²)_(z+z)HX

in which M, R², and z are as defined above for formula II, and X is ahalide anion.

Metal β-diketonates suitable for use in the present invention contain ametal atom and a β-diketone of formula III as a ligand:

in whichR³, R⁴, R⁵, and R⁶ are alike or different, and are independently chosenfrom hydrogen, unsubstituted and substituted alkyl, unsubstituted andsubstituted alkoxy, unsubstituted and substituted alkenyl, unsubstitutedand substituted alkynyl, unsubstituted and substituted heteroaryl,unsubstituted and substituted aryl, carboxylic acid groups, ester groupshaving unsubstituted and substituted alkyl, and combinations thereof,wherein substituted alkyl, alkoxy, alkenyl, alkynyl, heteroaryl, andaryl radicals are substituted with one or more alike or differentsubstituents independently chosen from halogen atoms, hydroxy, alkoxy,amino, heteroaryl, and aryl radicals.

It is understood that the β-diketone of formula III may assume differentisomeric and electronic configurations before and while chelated to themetal atom. For example, the free β-diketone may exhibit enolateisomerism. Also, the β-diketone may not retain strict carbon-oxygendouble bonds when the molecule is bound to the metal atom.

Examples of β-diketones useful in embodiments of the present inventioninclude acetylacetone, trifluoroacetylacetone, hexafluoroacetylacetone,2,2,6,6-tetramethyl-3,5-heptanedione,6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione, ethylacetoacetate, 2-methoxyethyl acetoacetate, benzoyltrifluoroacetone,pivaloyltrifluoroacetone, benzoyl-pyruvic acid, andmethyl-2,4-dioxo-4-phenylbutanoate.

Other ligands are possible on the metal β-diketonates useful in thepresent invention, such as, for example, alkoxides such as —OR² asdefined above, and dienyl radicals such as, for example,1,5-cyclooctadiene and norbornadiene.

Metal β-diketonates useful in the present invention can be madeaccording to any suitable method. β-diketones are well known aschelating agents for metals, facilitating synthesis of the diketonatefrom readily available metal salts.Metal β-diketonates are available from Alfa-Aesar and Gelest, Inc. Also,Strem Chemicals, Inc. of Newburyport, Mass., sells a wide variety ofmetal 3-diketonates on the internet athttp://www.strem.com/code/template.ghc?direct=cvdindex.

In some embodiments, a metal compound composition contains one metalcompound as its major component and one or more additional metalcompounds which may function as stabilizing additives. Stabilizingadditives, in some embodiments, comprise trivalent metal compounds.Trivalent metal compounds include, but are not limited to, chromium,iron, manganese and nickel carboxylates. A metal compound composition,in some embodiments, comprises both cerium and chromium carboxylates.

In some embodiments, the amount of metal forming the major component ofthe metal compound composition ranges from about 65 weight percent toabout 97 weight percent or from about 80 weight percent to about 87weight percent of the total metal in the compound composition. In otherembodiments, the amount of metal forming the major component of themetal compound composition ranges from about 90 weight percent to about97 weight percent of the total metal present in the compoundcomposition. In a further embodiment, the amount of metal forming themajor component of the metal compound composition is less than about 65weight percent or greater than about 97 weight percent of the totalmetal present in the compound composition.

In some embodiments, metal compounds operable to function as stabilizingadditives are present in amounts such that the total amount of the metalin metal compounds which are the stabilizing additives is at least 3% byweight of the total metal in the liquid metal compound composition.

The amount of metal in a liquid metal compound composition, according tosome embodiments, ranges from about 20 to about 150 grams of metal perkilogram of liquid metal compound composition. In other embodiments, theamount of metal in a liquid metal compound composition ranges from about30 to about 50 grams of metal per kilogram of liquid metal compoundcomposition. In a further embodiment, a liquid metal compoundcomposition comprises from about 30 to about 40 grams of metal per kg ofcomposition. In one embodiment, a metal amount is less than about 20grams of metal per kilogram of liquid metal compound or greater than 150grams of metal per kilogram of liquid metal compound.

Liquid metal compound compositions, in some embodiments of solid oxidecell production methods, further comprise one or more catalyticmaterials. Catalytic materials, in such embodiments, comprise transitionmetals including, but not limited to, platinum, palladium, rhodium,nickel, cerium, gold, silver, zinc, lead, ruthenium, rhenium, ormixtures thereof. Catalytic materials, in some embodiments, are presentin liquid metal compound compositions in an amount ranging from about0.5 weight percent to about 10 weight percent of the composition. Infurther embodiments, one or more catalytic materials are present in anamount of less than about 0.5 weight percent of the composition. Instill further embodiments, one or more catalytic materials are presentin an amount of greater than about 10 weight percent of the composition.In certain embodiments, the catalytic material is present in the liquidmetal compound composition in the form of a metal compound. In certainother embodiments, the catalytic material is present in the form of ametal.

In other embodiments, a liquid metal compound composition furthercomprises nanoparticles operable to alter the pore structure andporosity of the metal oxide resulting from the conversion of the liquidmetal compound composition. Nanoparticles, in some embodiments, comprisemetal oxide nanoparticles. Nanoparticles, in some embodiments, arepresent in liquid metal compound compositions in an amount ranging fromabout 0.5 percent by volume to about 30 percent by volume of the liquidmetal compound composition. In another embodiment, nanoparticles arepresent in the liquid metal compound composition in an amount rangingfrom about 5 percent by volume to about 15 percent by volume of theliquid metal compound composition.

In addition to liquids, metal compound compositions, in some embodimentsof the present invention, comprise solid metal compound compositions,vapor metal compound compositions, or combinations thereof.

In one embodiment, a solid metal compound composition comprises one ormore metal compound powders. In another embodiment, a vapor metalcompound composition comprises a gas phase metal compound operable tocondense on a substrate prior to conversion to a metal oxide. In someembodiments, the substrate is cooled to enhance condensation of thevapor phase metal compound composition. In one embodiment, for example,a substrate such as a steel electrode substrate is placed in a vacuumchamber, and the chamber is evacuated. Vapor of one or more metalcompounds, such as cerium (IV) 2-hexanoate, enters the vacuum chamberand deposits on the steel substrate. Subsequent to deposition, the metalcompound is exposed to conditions operable to convert the metal compoundto a metal oxide. In a further embodiment, a metal compound compositioncomprises gels chosen from suitable gels including, but not limited to,sol-gels, hydrogels, and combinations thereof.

Applying a metal compound composition to a substrate can be accomplishedby any suitable method, such as those known to one of skill in the art.In one embodiment, the substrate is dipped into the liquid metalcompound composition. In another embodiment, a swab, sponge, dropper,pipette, spray, brush or other applicator is used to apply the liquidmetal compound composition to the substrate. In some embodiments, avapor phase metal compound composition is condensed on the substrate. Inother embodiments, lithographic methods can be used to apply the metalcompound composition to the substrate.

A metal compound composition, in some embodiments, is applied to thesubstrate at a temperature less than about 250° C. In other embodiments,a metal compound composition is applied to the substrate at atemperature less than about 200° C., less than about 150° C., less thanabout 100° C., or less than about 50° C. In a further embodiment, ametal compound composition is applied to the substrate at roomtemperature. An additional embodiment provides a metal compoundcomposition applied at less than about room temperature.

A substrate onto which the at least one metal compound and optionallyone or more additional materials is applied is not limited. In someembodiments, the substrate is an electrode, while in other embodiments,the substrate is a thin sheet. In still other embodiments, a substrateis used only for forming the metal oxide electrolyte. After the metalcompound is converted to the metal oxide, the substrate in suchembodiments is removed.

A substrate, in some embodiments, is pretreated prior to application ofthe metal compound composition. In one embodiment, for example, thesubstrate can be etched according to known methods, for example, with anacid wash comprising nitric acid, sulphuric acid, hydrochloric acid,phosphoric acid, or a combination thereof, or with a base washcomprising sodium hydroxide or potassium hydroxide, for example. Inanother embodiment, the substrate is polished, with or without the aidof one or more chemical etching agents, abrasives, and polishing agents,to make the surface either rougher or smoother. In a further embodiment,the substrate is pretreated such as by carburizing, nitriding, plating,or anodizing.

Following application, the metal compound composition is at leastpartially converted to a metal oxide. In some embodiments, the metalcompound composition is fully converted to a metal oxide.

Converting a metal compound composition comprising a metal salt of acarboxylic acid, according to some embodiments of the present invention,comprises exposing the metal compound composition to an environmentoperable to convert the metal salt to a metal oxide. Environmentsoperable to convert metal compounds to metal oxides, in someembodiments, provide conditions sufficient to vaporize and/or decomposethe compound moieties and precipitate metal oxide formation. In oneembodiment, an environment operable to convert metal compounds to metaloxides comprises a heated environment. A metal salt of a carboxylicacid, for example, can be exposed to an environment heated to atemperature operable to convert the carboxylic acid and induce formationof the metal oxide. In some embodiments, the environment is heated to atemperature greater than about 200° C. In other embodiments, theenvironment is heated to a temperature greater than about 400° C. Incertain embodiments, the environment is heated to a temperature up toabout 425° C. or up to about 450° C. In additional embodiments, theenvironment is heated to a temperature ranging from about 400° C. toabout 650° C. In a further embodiment, the environment is heated to atemperature ranging from about 400° C. to about 550° C.

The rate at which the environment is heated to effect the conversion ofthe at least one metal compound to the at least one metal oxide is notlimited. In some embodiments, the heating rate is less than about 7°C./minute. In other embodiments, the heating rate is equal to about 7°C./minute. In still other embodiments, the heating rate is greater thanabout 7° C./minute. The heating rate, according to certain iterations ofthe present invention, is equal to the heating rate of the oven in whichthe conversion takes place. Particular embodiments provide a heatingrate that is as fast as the conditions and equipment allow.

In some embodiments, the metal oxide penetrates into the substrate to adepth ranging from about 10 nm to about 100 nm or from about 20 nm toabout 80 nm. In other embodiments, the metal oxide penetrates into thesubstrate to a depth ranging from about 30 nm to about 60 nm or fromabout 40 nm to about 50 nm. Converting the metal compound on thesubstrate to a metal oxide, in some embodiments, produces a transitionlayer comprising metal oxide and substrate material, in someembodiments. In other embodiments, the metal oxide does not penetrateinto the substrate and an abrupt interface exists between the metaloxide and the substrate.

Moreover, exposing metal compound compositions to environments operableto convert the compositions to metal oxides, as provided herein,eliminates or reduces the need for sintering to produce metal oxides. Byeliminating sintering, solid oxide cell production methods of thepresent invention gain several advantages. One advantage is that thelower temperatures of some methods of the present invention do notinduce grain growth or other degradative processes in various componentsof the solid oxide cell during production. Another advantage is that thecompound compositions permit tailoring of individual metal oxide layersin the construction of electrolytes and electrodes. Methods of thepresent invention, for example, permit one metal oxide layer of anelectrolyte or electrode to have completely different compositionaland/or physical parameters in comparison to an adjacent metal oxidelayer, in some embodiments. Such control over the construction ofelectrolytes and electrodes of solid oxide cells is extremely difficultand, in many cases, not possible with present sintering techniques. Inother embodiments, for example, one material can be prepared withconventional techniques such as sintering or epitaxial growth, while ametal oxide can be formed on that material without the need forsintering.

The conversion environment, for various embodiments of the presentinvention, can be any suitable environment, and the conversion can beprecipitated by any suitable means. In some embodiments of the presentinvention, the substrate is heated; in others, the atmosphere about themetal compound composition is heated; in still others, the metalcompound composition is heated. In further embodiments, a substratehaving a metal compound composition deposited thereon can be heated inan oven, or exposed to heated gas. The conversion environment may alsobe created using induction heating through means familiar to thoseskilled in the art of induction heating. Alternatively, the conversionenvironment may be provided using a laser applied to the surface areafor sufficient time to allow at least some of the metal compounds toconvert to metal oxides. In other applications, the conversionenvironment may be created using an infra-red light source which canreach sufficient temperatures to convert at least some of the metalcompounds to metal oxides. Some embodiments may employ a microwaveemission device to cause at least some of the metal compound to convert.Other embodiments provide a plasma to heat the metal compound. In thecase of induction heating, microwave heating, lasers, plasmas, and otherheating methods that can produce the necessary heat levels in a shorttime, for example, within seconds, 1 minute, 10 minutes, 20 minutes, 30minutes, 40 minutes, or one hour.

Further embodiments of the present invention relate to methods forforming a metal oxide electrolyte, comprising:

applying a metal compound to a first material in nanobar form; andconverting at least some of the metal compound to form a metal oxide,thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first material and of the metaloxide. Still other embodiments relate to electrolytes so formed, whereinthe nanobars conform to an orientation. That means that the greaterdimension (length) of at least a portion of the nanobars substantiallyalign in the same direction. Conforming to an orientation is caused, insome embodiments, by applying an orienting force before, during, or bothbefore and during the converting of the metal compound to the metaloxide. Certain embodiments supply an orienting force after theconverting as well. Orienting forces are not limited, and can be chosenfrom brushing, spin coating, one or more magnetic fields, one or moreelectric fields, and combinations thereof. In some embodiments, themagnetic field is chosen from static magnetic fields, variable magneticfields, uniform magnetic fields, non-uniform magnetic fields, andcombinations thereof.

Some devices for applying suitable magnetic fields appear, for example,U.S. Pat. No. 7,161,124 B2 to Kisner et al., which is incorporated byreference herein in its entirety. Devices for applying suitable magneticfields optionally provide one or more of heating, cooling, vacuum, fluidflushing, and manipulating means to the substrate being coated. Someembodiments provide a quartz vessel for holding one or more componentsto be coated in a magnetic field. Such a vessel, in some embodiments,contains one or more means for holding components so that evacuating,applying a magnetic field, heating, and cooling do not dislodge thecomponents. Such means for holding components include quartz structuresin the vessel that immobilize the components being coated. Care shouldbe taken so that components are not permitted to accelerate by theapplication of a large magnetic field. Quartz and similar materials thatare not affected by strong magnetic fields or higher temperatures aresuitable for some embodiments.

The magnetic field can be any suitable strength. In some embodiments,the magnetic field is less than one Tesla. In still further embodiments,the magnetic field ranges from about 1 Tesla to about 2 Tesla, fromabout 2 Tesla to about 4 Tesla, from about 4 Tesla to about 6 Tesla,from about 6 Tesla to about 8 Tesla, from about 8 Tesla to about 10Tesla, or greater than about 10 Tesla.

In other embodiments, the electric field is chosen from static electricfields, variable electric fields, uniform electric fields, non-uniformelectric fields, and combinations thereof. For example, two largeconductive plates arranged like a parallel plate capacitor can provide asubstantially uniform electric field. Into the field is placed asubstrate comprising at least one metal compound and at least onenanobar, in some embodiments, and the temperature is raised to effectconversion of the metal compound to metal oxide while under the effectof the electric field. Optionally, the electrodes that will form thecell can be charged to create an electric field. Or a corona polingarrangement can be made, in which a charged needle provides an electricfield and scans across the substrate having thereon at least one metaloxide and at least one kind of nanobar. Scanning with the needle is ameans for converting the metal compound into metal oxide, such as, forexample, one or more laser diodes or a mirror directing a laser beam tothe region where the electric field is strongest. In that manner, theconversion of the metal compound to form the metal oxide would lock inthe orientation of the nanobars provided by the charged needle.

To establish an electric field, a device capable of applying andmaintaining a high voltage difference across two electrodes is needed.The Slaughter Company, of Lake Forest, Ill. (www.hipot.com) offersseveral “hipot” or high potential instruments providing up to 6000 V ACor DC. In certain embodiments, at least one metal compound and at leastone nanobar are applied to an electrode to be used in a cell, andanother electrode to be used in the cell is positioned substantiallyparallel to the first electrode. Optionally, the second electrode isclose enough to touch the at least one metal compound; but care is takento avoid shorting the two electrodes. An electric potential is appliedacross the two electrodes and the resulting field orients at least aportion of the nanobars, and the metal compound is heated to convertinto the metal oxide, such as, for example by an oven containing the twoelectrodes.

Further embodiments provide a nanobar having one or more alike ordifferent derivatives. For example, a nanobar can be chemicallyfunctionalized at the bar end, at the sidewall, or a combinationthereof. Tube end functionalization, in certain embodiments, facilitatesthe addition of one or more ionic or non-ionic species that can assistin orienting the nanobar in an electric or magnetic field. Tube end andsidewall functionalization can be obtained, for example, by reactingcarbon nanotubes with diazonium species as described in U.S. Pat. No.7,250,147, which patent is incorporated herein by reference in itsentirety. Accordingly, in one embodiment, a benzenediazoniumtetrafluoroborate salt para-substituted with a chosen functional groupis attached to single-wall carbon nanotubes by holding a bucky paperworking electrode comprising the nanotubes at −1.0 V vs Ag/AgNO₃ in asolution of the salt for 30 minutes. The nanotubes so functionalized arethen mixed with metal compound, applied to a substrate, oriented in amagnetic or electric field, or by brushing or spin-coating, and themetal compound is converted to form the metal oxide about thefunctionalized nanotube.

In a further embodiment, 4-hydroxycarbonylphenyldiazoniumtetrafluoroborate functionalizes single-wall carbon nanotubes inaccordance with the '147 patent. Then, one or more metal ions are addedto the carboxylate groups, for example, by rinsing with mild basicsolution to deprotonate the carboxylate groups, and then one or morealike or different metal salts are introduced. The nanotubesfunctionalized with metal carboxylates are dispersed on a substrate,optionally with one or more alike or different metal compounds, and theenvironment is heated to form one or more metal oxides from the metalions on the nanotubes and optional metal compounds. In some embodiments,the nanotubes are oriented by brushing, spin coating, or by applying amagnetic or electric field, or by a combination of any of the foregoing.In certain embodiments, oriented domains of metal oxide are formed. Inother embodiments, an electrolyte comprising oriented domains of metaloxide appear, wherein the electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the metal oxide.

Nanobars, such as carbon nanotubes can be functionalized for example byreacting with fluorine gas, and optionally further reacting with one ormore nucleophilic species as set forth in U.S. Patent ApplicationPublication No. US2002/0004028, which is incorporated herein byreference in its entirety. U.S. Patent Application Publication No.US2005/0089684 discloses the deposition of inorganic oxides such assilica on carbon nanotubes optionally functionalized for example withhydroxyl groups. Once the nanotubes are at least partially coated withsilica, the coating process is stopped and the nanotubes can bedeposited on a substrate, for example, for microelectronic devicefabrication. The '684 publication is incorporated herein by reference inits entirety. US Patent Application Publication No. 2008/0233040, whichis also incorporated herein by reference in its entirety, describesfunctionalizing the silica coating of silica-coated nanotubes. K.Hernadi et al., “Synthesis of MWNT-based Composite Materials withInorganic Coating,” Acta Mater., 51 (2003) 1447, discloses formingalumina, silica, and titania on multi-walled carbon nanotubes usingmetal alkoxide compounds. The Hernadi article is incorporated byreference herein in its entirety.

Some embodiments of the present invention provide a method for making ametal oxide electrolyte, comprising applying a nanobar functionalizedwith a metal compound to a substrate, optionally orienting the nanobar,and converting the metal compound to a metal oxide, thereby forming themetal oxide electrolyte; wherein the metal oxide electrolyte has anionic conductivity greater than the bulk ionic conductivity of the metaloxide. Further embodiments relate to the metal oxide electrolyte somade, while even further embodiments relate to a solid oxide cellcomprising a metal oxide electrolyte so made. Optionally, a metalcompound is applied to the substrate before, during, and/or after theapplying of the nanobar, and that metal compound can be the same ordifferent from the metal compound functionalizing the nanobar. Incertain embodiments, the metal oxide electrolyte comprises the nanobar.In other embodiments, the nanobar does not appear, in some cases becausethe conversion conditions have destroyed the nanobar. Furtherembodiments provide the metal oxide in oriented domains.

In another embodiment, nanobars such as inorganic nanorods or carbonnanotubes chosen from single wall nanotubes, multiwall nanotubes, andcombinations thereof are contacted with one or more alike or differentmetal compounds, applied to a substrate, optionally orienting thenanobars, and converting at least some of the metal compound to formmetal oxide, thereby forming a metal oxide electrolyte having an ionicconductivity greater than the bulk ionic conductivity of the metaloxide. In some embodiments, the applying action orients the nanobars,such as brushing or spin coating. In other embodiments, one or moreseparate orienting steps are taken, such as, for example, brushing, spincoating, exposing the nanobars to an electric field or a magnetic field,or a combination thereof. In certain cases, the nanobar remains in themetal oxide electrolyte, while in other cases, the nanobar is partiallyor completely absent, for example due to reaction, decomposition,sublimation, or the like. Additional embodiments provide pairs of metalcompounds in any ratio chosen from yttrium and zirconium, samarium andcerium, barium and titanium, strontium and titantium, and combinationsthereof.

In yet another embodiment, a metal compound is applied to mica flakes toform a mixture, and the mixture is applied to a planar electrode.Another planar electrode is placed over the mixture on the firstelectrode, an electric field established by the two electrodes, and themetal compound is converted to form the metal oxide. Optionally, themica flakes are preselected for susceptibility to orient in an electricfield. One method to preselect involves sorting a collection of micaflakes in an electric field, whereby those mica flakes that are affectedby the electric field are separated from those mica flakes that showlittle or no effect from the electric field. In another embodiment, micaflakes are pretreated, such as, for example, by contacting with acid orwith base, and then optionally preselected for susceptibility to orientin an electric field. In still other embodiments, mica flakes arepreselected for susceptibility to orient in a magnetic field, optionallyfollowing contact with acid or with base. Without wishing to be bound bytheory, it is believed that contact with acid or with base modifies thesurface properties such as surface charge, allowing the mica flake toorient in an electric field or magnetic field.

Accordingly, further embodiments provide applying an orienting force toa first material in powder form before, during, or before and during theconverting of the metal compound to the metal oxide. In someembodiments, the orienting force is chosen from magnetic fields,electric fields, and combinations thereof.

Further embodiments provide sequential formation of two or more metaloxides to form a metal oxide electrolyte. For example, a first metalcompound is applied to a substrate such as an electrode, and convertedto a first metal oxide. Depending on the amount of metal compound andthe manner of application, the resulting first metal oxide is porous, insome embodiments. Then, a second metal compound is applied to thesurface having the first metal oxide, and converted to a second metaloxide. Successive domains of first metal oxide and second metal oxideare formed on the surface by repeatedly applying and converting therespective metal compounds. In that way, a metal oxide electrolyte canbe built on the substrate so that multiple interfaces between the firstmetal oxide and second metal oxide form. Depending on the amount, or ifpresent in a composition, the concentration, of the metal compounds, theresulting metal oxide domains can have pores, voids, or discontinuities.Those defects can allow the penetration of subsequently applied metalcompound into the metal oxide, and give rise to interfaces between theoxides that run roughly perpendicularly from the surface of thesubstrate. Without wishing to be bound by theory, those verticalinterfaces can give rise to crystal structure defects between the twooxides and enhance ionic conductivity. In some embodiments, asuperlattice can be formed of alternating interpenetrating layers ofmetal oxides.

Accordingly, some embodiments provide a method for forming a metal oxideelectrolyte, comprising:

applying a first metal compound to a substrate;converting at least some of the first metal compound to form a firstmetal oxide on the substrate; applying a second metal compound to thesubstrate comprising the first metal oxide; andconverting at least some of the second metal compound to form a secondmetal oxide on the substrate comprising the first metal oxide,thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first metal oxide and of thesecond metal oxide. Further embodiments provide applying additionalfirst metal compound to the substrate comprising the first metal oxideand the second metal oxide; and converting at least some of theadditional first metal compound to form additional first metal oxide.

Still other embodiments of the present invention relate to applyingadditional second metal compound to the additional first metal oxide;and converting at least some of the additional second metal compound toform additional second metal oxide.

In some embodiments, metal oxides suitable for metal oxide electrolytescomprise zirconium oxides combined with various transition and/or rareearth metals, including, but not limited to, scandium, yttrium, erbium,ytterbium, europium, gadolinium, or dysprosium, or combinations thereof.In one embodiment, a metal oxide suitable for one or more layers of anelectrolyte comprises zirconium oxide (ZrO₂) or yttria-stabilizedzirconia (YSZ) Zr_((1−x))Y_(x)O_([2−(x/2)]), x=0.08-0.20, or 0.10-0.50,or 0.15-0.20, in certain embodiments. In another embodiment, a suitableelectrolyte metal oxide comprises scandia-stabilized zirconia (SSZ)Zr_((1−x))Sc_(x)O_([2−(x/2)]), x=0.09-0.11. Additional suitableelectrolyte zirconium compounds comprise zirconium silicate (ZrSiO₄),Zr_(0.85)Ca_(0.15)O_(1.85) or 3ZrO₂2CeO₂+10% CaO.

In another embodiment, metal oxides of an electrolyte comprise ceriumoxides of the general formula Ce_((1-x))M_(x)O_((2-δ)), x=0.10-0.20, andδ=x/2. In some embodiments M is samarium or gadolinium to produceCeO₂—Sm₂O₃ or CeO₂—Gd₂O₃.

Additional metal oxides suitable for electrolytes of solid oxide cellsof the present invention, comprise perovskite structured metal oxides.In some embodiments, perovskite structured metal oxides compriselanthanum gallates (LaGaO₃). Lanthanum gallates, in some embodiments,are doped with alkaline earth metals or transition metals, orcombinations thereof. In another embodiment, a perovskite structuremetal oxide comprises lanthanum strontium gallium magnesium oxide (LSGM)La_((1−x))Sr_(x)Ga_((1−y))Mg_(y)O_((3−δ)), x=0.10-0.20, y=0.15-0.20, andδ=(x+y)/2.

In a further embodiment, metal oxides suitable for electrolytes comprisebrownmillerites, such as barium indiate (Ba₂In₂O₆), non-cubic oxidessuch as lanthanum silicate, neodymium silicate, or bismuth based oxide,or combinations thereof.

Electrolytes of solid oxide cells, according to some embodiments of thepresent invention, comprise a plurality of nanocrystalline grains, thenanocrystalline grains comprising one or more of the metal oxides thatare suitable for use as an electrolyte in a solid oxide cell. In someembodiments, the nanocrystalline grains have an average size of lessthan about 50 nm. In other embodiments, nanocrystalline grains ofelectrolyte layers have an average size ranging from about 2 nm to about40 nm or from about 3 nm to about 30 nm. In another embodiment,nanocrystalline grains have an average size ranging from about 10 nm toabout 25 nm. In a further embodiment, nanocrystalline grains have anaverage size less than about 10 nm or less than about 5 nm.

Electrolytes of solid oxide cells are substantially non porous, in someembodiments. In one embodiment, an electrolyte has a porosity less thanabout 20%. In another embodiment, an electrolyte has a porosity lessthan about 15% or less than about 10%. In a further embodiment, anelectrolyte has a porosity less than about 5% or less than about 1%. Inone embodiment, an electrolyte is fully dense meaning that theelectrolyte has no porosity.

Once the metal oxide is formed, in some embodiments of the presentinvention, one or more epoxies can be applied to the metal oxide. Inaddition, or alternatively, epoxy can be applied to other components,such as one or more electrodes of the solid oxide cell. Epoxy can beused, in some embodiments of the present invention, to seal the solidoxide cell so that reactants from one side of the cell do not penetrateto the other side of the cell. Any suitable epoxy that can withstand theoperating temperature of the solid oxide cell can be used alone or incombination. U.S. Pat. No. 4,925,886 to Atkins et al. discloses andclaims epoxy compositions comprising two epoxies and having a usabletemperature of at least 160° C., for example. U.S. Pat. No. 6,624,213 toGeorge et al. reports tests of various epoxy compositions at 177° C.,for further examples. The '886 patent and the '213 patent areincorporated by reference herein in their entireties.

In some embodiments, an electrolyte has a thickness (distance between acathode and an anode) ranging from about 1 nm to about 1 mm or fromabout 10 nm to about 500 μm. In other embodiments, an electrolyte has athickness ranging from about 50 nm to about 250 μm, from about 100 nm toabout 100 μm, or from about 500 nm to about 50 μm. In anotherembodiment, an electrolyte has a thickness ranging from about 750 nm toabout 10 μm, or from about 1 μm to about 5 μm, or from about 1.2 μm toabout 4 μm, or from about 1.5 μm to about 2 μm. In a further embodiment,an electrolyte has a thickness less than about 10 μm or less than about1 μm. In one embodiment, an electrolyte has a thickness ranging fromabout 1 nm to about 100 nm or from about 50 nm to about 100 nm. In stillother embodiments, an electrolyte has a thickness greater than about 500μm.

Materials suitable for use in air electrodes, fuel electrodes,electrolyzer electrodes, sensors, and/or electrolytes, in addition tothe materials recited hereinabove, can be chosen from CeO₂—ZrO₂ whereinCeO₂ is about 10-90 weight percent; yttria-stabilized zirconia (YSZ)wherein yttria is present in an amount of about 1-50 mol percent;CeO₂—PrO₂ wherein PrO₂ is up to about 50 weight percent; PrO₂—CeO₂—ZrO₂wherein PrO₂—CeO₂ is up to about 90 weight percent; PrO₂—ZrO₂ whereinPrO₂ is 10 to 90 weight percent; scandia-doped zirconia (SSZ) doped withone or more of CeO₃O₄, Bi₂O₃, SiO₂, TiO₂, Fe₂O₃, NiO, MnO₂, CeO₂, andAl₂O₃; YSZ doped with one or more of Co₃O₄, Bi₂O₃, SiO₂, TiO₂, Fe₂O₃,NiO, MnO₂, CeO₂, and Al₂O₃; CaO stabilized zirconia doped with one ormore of Co₃O₄, Bi₂O₃, SiO₂, TiO₂, Fe₂O₃, NiO, MnO₂, CeO₂, and Al₂O₃;mixed LSM and YSZ; and combinations thereof. The relative amounts of thevarious oxides are not limited. In some embodiments, for example, YSZcomprises about 8 mole percent Al₂O₃. In other embodiments, about 30mole percent Al₂O₃ is present. In still further embodiments, about 90mole percent Al₂O₃ appears. In yet another embodiment, a metal oxidecomprises cerium, samarium, and oxygen in the approximate mole ratios0.85:0.15:1.925. An additional embodiment provides cerium, gadolinium,and oxygen in the approximate mole ratios of 0.9:0.1:1.95.

Oxides of the following elements can be used in embodiments of airelectrodes, fuel electrodes, electrolyzer electrodes, sensors, and/orelectrolytes in some embodiments of the present invention: lithium,beryllium, sodium, magnesium, aluminum, silicon, potassium, calcium,scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel,copper, zinc, gallium, germanium, arsenic, bromine, rubidium, strontium,yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium,palladium, antimony, tellurium, silver, cadmium, indium, tin, cesium,barium, lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium, lutetium, hafnium, tantalum, tungsten, rhenium,osmium, iridium, gold, mercury, thallium, lead, bismuth, radium,actinium, platinum, thorium, protactinium, uranium, neptunium,plutonium, americium, berkelium, californium, einsteinium, fermium,mendelevium, nobelium, lawrencium, or curium. Oxides containing morethan one of the foregoing elements, and oxides containing elements inaddition to the foregoing elements, also can be used in embodiments ofthe present invention. For example, alumina containing small amounts ofchromium, titanium, iron, vanadium, and combinations thereof, akin tothe mineral corundum and gemstones sapphire and ruby, can be used incertain embodiments.

Moreover, in some embodiments, one or more catalytic materials can beincorporated into each of the foregoing metal oxide materials in anamount ranging from about 0.5 to about 10 weight percent. In otherembodiments, one or more catalytic materials can be incorporated in anamount less than about 5 weight percent. In still other embodiments, oneor more catalytic materials can be incorporated in an amount greaterthan about 10 weight percent.

In some embodiments of solid oxide cells of the present invention, anelectrode-electrolyte transition layer is interposed between theelectrolyte and the electrode. An electrode-electrolyte transition layercomprises both electrode and electrolyte materials. By comprising bothelectrode and electrolyte materials, the electrode-electrolytetransition layer, in some embodiments, is operable to reduce disparitiesin coefficients of thermal expansion between the electrode andelectrolyte. Reducing such disparities can have an inhibitory effect ondegradative pathways such as cracking or delamination between theelectrode and electrolyte. Moreover, an electrode-electrolyte transitionlayer provides increased stability by anchoring the electrolyte to theelectrode. The electrode-electrolyte transition layer, in someembodiments, additionally provides a robust base on which to furtherbuild an electrolyte, the electrolyte having thickness less than about10 μm or less than about 1 μm, in some cases.

In some embodiments, an electrode-electrolyte transition layer has athickness ranging from about 1 nm to about 5 nm, from about 5 nm toabout 10 nm, from about 10 nm to about 20 nm, from about 20 nm to about50 nm, from about 50 nm to about 100 nm or from about 20 nm to about 80nm. In another embodiment, an electrode-electrolyte transition layer hasa thickness ranging from about 30 nm to about 60 nm or from about 40 nmto about 50 nm. In a further embodiment, an electrode-electrolytetransition layer has a thickness less than about 10 nm or greater thanabout 100 nm.

FIG. 1 is a micrograph at approximately two million×magnificationillustrating an electrode-electrolyte transition layer according to oneembodiment of the present invention. In the micrograph, a YSZelectrolyte (102) is disposed on an electrode substrate (104) made ofstainless steel 304. An electrode-electrolyte interlayer (106) isinterposed between the YSZ electrolyte (102) and the electrode substrate(104).

Electrodes

Electrodes of the present invention, in some embodiments, comprise asubstrate. In some embodiments, a substrate comprises silicon carbidedoped with titanium. In other embodiments, a substrate comprisesLa_(1−x)Sr_(x)MnO₃ [lanthanum strontium doped manganite (LSM)]. Inanother embodiment, a substrate comprises one or more porous steelalloys. In one embodiment, a porous steel alloy comprises steel alloy52. In some embodiments, a porous steel alloy suitable for use as anelectrode substrate comprises steel alloy 316, stainless steel alloy430, Crofer 22 APU® (Thyssen Krupp), E-Brite® (Alleghany Ludlum),HASTELLOY® C-276, INCONEL® 600, or HASTELLOY® X, each of which iscommercially available from Mott Corporation of Farmington, Conn. Yetadditional embodiments provide an electrode substrate comprising nickelsuch as, for example, Nickel Alloy 200. Certain embodiments employ anelectrode comprising porous graphite, optionally with one or morecatalytic materials. In a further embodiment, a substrate comprises anymetal or alloy known to one of skill in the art operable to serve as anelectrode. Some embodiments of the present invention provide electrodesubstrates comprising a metal, a metal carbide, or a combinationthereof. Certain additional embodiments provide an electrode substratecomprising titanium silicate carbide. In some of those embodiments, theelectrode substrate material may have electrical, structural, andmechanical properties that are better than those of ceramic electrodes.

Electrode substrates, according to further embodiments of the presentinvention, are porous. In some embodiments, a substrate has a porosityranging from about 5% to about 40%. In another embodiment, a substratehas a porosity ranging from about 10% to about 30% or from about 15% toabout 25%. In a further embodiment, a substrate has a porosity greaterthan about 40%. A substrate, in some embodiments, has a porosity rangingfrom about 40% to about 80%. In one embodiment, a substrate has aporosity greater than about 80%.

In addition to a substrate, some electrodes of the present inventionoptionally comprise a coating disposed on the substrate, the coatingcomprising at least one layer of at least one metal oxide. In someembodiments, a coating disposed on the substrate comprises a pluralityof layers comprising one or more metal oxides. Metal oxide layerssuitable for use in electrodes of the present invention can comprise anyof the metal oxides recited herein, including cerium samarium oxides.Some embodiments of the present invention provide a metal oxide coatingdisposed on the electrode substrate that can act as an electrolyte, anelectrode-electrolyte transition layer, a concentration-gradient layer,a matching layer for coefficients of thermal expansion, an electricalinsulator, or a combination thereof, among other functions.

Substrate coatings comprising one or more metal oxide layers, accordingto some embodiments of the present invention, are porous. In oneembodiment, a coating has a porosity ranging from about 5% to about 40%.In another embodiment, a coating has a porosity ranging from about 10%to about 30% or from about 15% to about 25%. In a further embodiment, asubstrate coating has a porosity greater than about 40%. A substratecoating, in some embodiments, has a porosity ranging from about 40% toabout 60%. In one embodiment, a substrate coating has a porosity greaterthan about 60%.

Substrate coatings can have any desired thickness. In one embodiment asubstrate coating has a thickness ranging from about 1 nm to about 1micron. In another embodiment, a substrate coating has a thicknessranging from about 50 nm to about 750 μm, from about 500 nm to about 500μm, from about 1 μm to about 350 μm, or from about 10 μm to about 200μm. In a further embodiment, a substrate coating has a thickness rangingfrom about 50 μm to about 100 μm. In some embodiments wherein a coatingcomprises a plurality of metal oxide layers, each metal oxide layer hasa thickness ranging from about 5 nm to about 15 nm, wherein the totalthickness of the coating is the summation of the thicknesses of theindividual layers.

In some embodiments of electrodes of the present invention, asubstrate-coating transition layer is interposed between the substrateand the coating. A substrate-coating transition layer comprises bothsubstrate and coating materials. By comprising both substrate andcoating materials, the substrate-coating transition layer, in someembodiments, is operable to reduce disparities in coefficients ofthermal expansion between the substrate and the metal oxide coating ofthe electrode. Reducing such disparities can have an inhibitory effecton degradative pathways such as cracking or delamination between thesubstrate and metal oxide coating. Moreover, a substrate-coatingtransition layer provides increased stability by anchoring the metaloxide coating to the electrode.

In some embodiments, a substrate-coating transition layer has athickness ranging from about 3 nm to about 100 nm or from about 20 nm toabout 80 nm. In another embodiment, a substrate-coating transition layerhas a thickness ranging from about 30 nm to about 60 nm or from about 40nm to about 50 nm. In a further embodiment, a substrate-coatingtransition layer has a thickness less than about 10 nm or greater thanabout 100 nm.

Electrodes, according to some embodiments of the present invention,further comprise catalytic materials. Catalytic materials can comprisetransition metals including, but not limited to, platinum, palladium,rhodium, nickel, cerium, gold, silver, zinc, lead, ruthenium, rhenium,or mixtures thereof. Catalytic materials, in some embodiments, aredisposed in one or a plurality of metal oxide layers coating thesubstrate of an electrode. The combination of a metal oxide with puremetals or alloys, in some embodiments, produces a cermet. Electrodes ofsolid oxide fuel cells further comprising catalytic materials canfunction as fuel reformers operable to convert hydrocarbon fuels intohydrogen for subsequent use in the solid oxide fuel cell, in someembodiments. Moreover, electrodes further comprising catalytic materialscan function as fuel reformers upstream and independent from the solidoxide fuel cell in other embodiments.

Electrodes comprising catalytic materials can additionally demonstratecompositional gradients based on the distribution of the catalyticmaterials in the plurality of metal oxide layers. In one embodiment, anelectrode comprises a substrate and a plurality of metal oxide layersdisposed on the substrate, wherein metal oxide layers closer to thesubstrate comprise greater amounts of catalytic material than metaloxide layers further from the substrate. Moreover, in anotherembodiment, metal oxide layers further from the substrate comprisegreater amounts of catalytic material than metal oxide layers closer tothe substrate. In one embodiment, for example, metal oxide layersfurther from the substrate comprise about 5 weight percent catalyticmaterial while metal oxide layers closer to the substrate comprise about1 weight percent catalytic material.

Electrodes of the present invention, in some embodiments, are resistantto harsh environments and various chemical species which can foul theelectrodes, such as sulfur or carbon. An electrode, in one embodiment,is an anode. An electrode, in another embodiment, is a cathode. In someembodiments, the metal oxide coating of an electrode can protect theelectrode substrate from corrosion and/or degradation.

Turning now to components that can be included in solid oxide fuelcells, solid oxide fuel cells of the present invention comprise an airelectrode. The air electrode of a solid oxide fuel cell operates as acathode to reduce oxygen molecules thereby producing oxygen anions forsubsequent transport through the electrolyte. In some embodiments, anair electrode comprises p-type semiconducting oxides such as lanthanummanganite (LaMnO₃). Lanthanum manganite can be doped with rare earthelements, such as strontium, cerium, and/or praseodymium to enhanceconductivity. In one embodiment, an air electrode comprisesLa_(1−x)Sr_(x)MnO₃ [lanthanum strontium doped manganite (LSM)]. Inanother embodiment, an air electrode comprises lanthanum strontiumferrite or lanthanum strontium cobaltite or a combination thereof.

Air electrodes, according to some embodiments of the present invention,are porous. In one embodiment, an air electrode has a porosity rangingfrom about 5% to about 30%. In another embodiment, an air electrode hasa porosity ranging from about 10% to about 25% or from about 15% toabout 20%. In a further embodiment, an air electrode has a porositygreater than about 30%. An air electrode, in some embodiments, has aporosity ranging from about 30% to about 60% or from about 40% to about80%. In one embodiment, an air electrode has a porosity greater thanabout 80%.

In addition to an air electrode, a solid oxide fuel cell comprises afuel electrode. A fuel electrode, in some embodiments, comprises one ormore metal oxides combined with one or a plurality of catalyticmaterials. Catalytic materials, as provided herein, comprise transitionmetals including, but not limited to, platinum, palladium, rhodium,nickel, cerium, gold, silver, zinc, lead, ruthenium, rhenium, ormixtures thereof. In one embodiment, a fuel electrode comprises zirconia(ZrO₂) combined with Ni. Yttria-stabilized zirconia (YSZ),Zr_((1−x))Y_(x)O_([2−(x/2)]), for example, can be combined with Ni toproduce a Ni-YSZ fuel electrode. Catalytic materials, in someembodiments, are incorporated into metal oxide compositions of fuelelectrodes in an amount ranging from about 0.5 to about 10 weightpercent. In other embodiments, catalytic materials are incorporated intometal oxide compositions of fuel electrodes in an amount less than about5 weight percent, less than about 0.5 weight percent, or greater thanabout 10 weight percent.

Fuel electrodes, according to some embodiments of the present invention,are porous. In one embodiment, a fuel electrode has a porosity rangingfrom about 5% to about 40%. In another embodiment, a fuel electrode hasa porosity ranging from about 10% to about 30% or from about 15% toabout 25%. In a further embodiment, a fuel electrode has a porositygreater than about 40%. A fuel electrode, in some embodiments, has aporosity ranging from about 40% to about 80%. In still otherembodiments, a fuel electrode has a porosity greater than about 80%.

In order to reduce problems and disadvantages associated with variancesin coefficients of thermal expansion between electrode and electrolytesof solid oxide cells, electrodes, in some embodiments of the presentinvention, comprise compositional gradients. An electrode, in oneembodiment, comprises a region closer to the electrolyte and a regionfurther from the electrolyte, wherein the region closer to theelectrolyte comprises a greater amount of electrolyte material than theregion of the electrode further from the electrolyte. In anotherembodiment, an electrode of a solid oxide cell comprises a plurality oflayers. Layers of the electrode closer to the electrolyte comprisegreater amounts of electrolyte material than layers of the electrodefurther from the electrolyte. A solid oxide cell, in some embodiments,comprises a first electrode comprising a plurality of layers of a firstmaterial and an electrolyte comprising an electrolyte material disposedon the first electrode, wherein layers of the first material closer toor adjacent to the electrolyte further comprise greater amounts of theelectrolyte material than layers of the first material further from orspaced apart from the electrolyte.

In addition to variances in coefficients of thermal expansion, some ofthe solid oxide cells of the present invention also address fuel, air,and other reactant delivery mechanisms by providing electrodescomprising porosity and optionally, porosity gradients. For example,electrodes of solid oxide fuel cells may be porous in order to allow theingress of air and fuel to the electrolyte and the egress of other gasesproduced or not consumed by the fuel cell. In one embodiment, a solidoxide cell comprises a solid electrolyte disposed on a first electrode,the first electrode comprising a first region closer to the solidelectrolyte and a second region further from the electrolyte, whereinthe first region has a porosity less than the second region.Alternatively, in another embodiment, the first region of the electrodehas a porosity that is greater than the second region of the electrode.

Moreover, in some embodiments, a solid oxide cell comprises a firstelectrode comprising a plurality of layers of a first material and asolid electrolyte disposed on the first electrode, wherein layers of thefirst material closer to the solid electrolyte have porosities less thanlayers of the first material further from the solid electrolyte.Alternatively, in other embodiments, layers of the first material closerto the solid electrolyte have porosities greater than layers of thefirst material further from the solid electrolyte.

Interconnects

In another aspect, the present invention provides interconnects operableto be used in solid oxide cells as well as other applications.Interconnects of the present invention, in some embodiments, areresistant to harsh environments and chemical species which can degradethe interconnects. In one embodiment, the present invention provides aninterconnect comprising a substrate comprising a first material, acoating composition comprising a layer of a metal oxide disposed on thesubstrate, and optionally a substrate-coating transition layerinterposed between the substrate and the coating. In some embodiments, acoating composition comprises a plurality of metal oxide layers. One ora plurality of metal oxide coatings can assist in protecting a metallicor ceramic interconnect substrate from degradative conditions and/orchemical species.

Interconnects, in some embodiments, comprise substrates. In certainembodiments, substrates comprise metal oxides including, but not limitedto, lanthanum and yttrium chromites. In other embodiments, a substratecomprises metals or alloys, such as chromium based alloys. In oneembodiment, a chromium based alloy comprises 5 weight percent iron and 1weight percent yttria. In another embodiment, a substrate comprises aferritic steel. In a further embodiment, a substrate comprises any metaloperable to sufficiently transfer charge carriers into or from anexternal circuit. Thus, in some embodiments, interconnects of thepresent invention are adaptable to provide electrical communicationbetween a electrode and an external circuit. In further embodiments,interconnects are adaptable to provide material communication between anelectrode and an external source of a material, and/or an exit for amaterial. For example, an interconnect can provide air or oxygen to thecathode of a solid oxide fuel cell. For another example, an interconnectcan provide an exhaust conduit for water or steam to exit a solid oxidefuel cell. For yet another example, an interconnect can provide aconduit to a storage system for hydrogen generated at the cathode of asolid oxide electrolyzer cell. In still other embodiments, aninterconnect provides both electrical and material communication betweena electrode and an external circuit and external sources and/orreservoirs and/or exhaust for material. Optionally, an interconnect maybe adapted to provide thermal communication between an electrode and anexternal source or sink for thermal energy.

Accordingly, interconnects can have any desired shape. Wires, films,monoliths, porous monoliths, disks, tubes, pipes, among other shapes,are possible. Connections between an interconnect and an electrode canadopt any suitable form. In some embodiments, the same piece of metal(or metal carbide, cermet, or other material) forms the substrate forthe electrode and for the interconnect; in such embodiments, the portionof the metal that engages in the electrochemical reaction in the cell isthe electrode portion, while the portion of the metal providingcommunication outside the cell is the interconnect portion. In otherembodiments, electrical contact between the interconnect and theelectrode are made with any suitable connection, such as, for example,welding, stamping, melt fusion, mechanical connections such as bolts orrivets, conductive paints such as silver paint, sputtered metals, andconductive adhesives, as well as combinations thereof. Such connectionscan be made before, during, or after formation of metal oxides asdescribed herein.

Interconnect substrates can have any desired thickness. In oneembodiment a substrate has a thickness ranging from about 1 nm to about1 mm. In another embodiment, a substrate has a thickness ranging fromabout 50 nm to about 750 μm, from about 500 nm to about 500 μm, fromabout 1 μm to about 350 μm, or from about 10 μm to about 200 μm. In afurther embodiment, a substrate has a thickness ranging from about 50 μmto about 100 μm.

In addition to a substrate, an interconnect of some embodiments of thepresent invention comprises a coating disposed on the substrate, thecoating comprising a layer of a metal oxide. In some embodiments, acoating disposed on the substrate comprises a plurality of layerscomprising one or more metal oxides. Metal oxide layers suitable for usein interconnects of the present invention can comprise any of the metaloxides recited herein, such as, for example, any of the cerium samariumoxides.

Substrate coatings comprising one or more metal oxide layers, accordingto some embodiments of the present invention, are porous. In oneembodiment, a coating has a porosity ranging from about 5% to about 40%.In another embodiment, a coating has a porosity ranging from about 10%to about 30% or from about 15% to about 25%. In a further embodiment, asubstrate coating has a porosity greater than about 40%. A substratecoating, in some embodiments, has a porosity ranging from about 40% toabout 60%. In one embodiment, a substrate coating has a porosity greaterthan about 60%.

Substrate coatings can have any desired thickness. In one embodiment, asubstrate coating has a thickness ranging from about 1 nm to about 1micron. In another embodiment, a substrate coating has a thicknessranging from about 50 nm to about 750 μm, from about 500 nm to about 500μm, from about 1 μm to about 350 μm, or from about 10 μm to about 200μm. In a further embodiment, a substrate coating has a thickness rangingfrom about 50 μm to about 100 μm. In some embodiments wherein a coatingcomprises a plurality of metal oxide layers, each metal oxide layer hasa thickness ranging from about 5 nm to about 15 nm, wherein the totalthickness of the coating is the summation of the thicknesses of theindividual layers.

In some embodiments of interconnects of the present invention, asubstrate-coating transition layer is interposed between the substrateand the coating. A substrate-coating transition layer comprises bothsubstrate and coating materials. By comprising both substrate andcoating materials, the substrate-coating transition layer, in someembodiments, is operable to reduce disparities in coefficients ofthermal expansion between the substrate and the metal oxide coating ofthe interconnect. Reducing such disparities can have an inhibitoryeffect on degradative pathways such as cracking or delamination betweenthe substrate and metal oxide coating. Moreover, a substrate-coatingtransition layer provides increased stability by anchoring the metaloxide coating to the electrode.

In some embodiments, a substrate-coating transition layer of aninterconnect has a thickness ranging from about 3 nm to about 100 nm orfrom about 20 nm to about 80 nm. In another embodiment, asubstrate-coating transition layer has a thickness ranging from about 30nm to about 60 nm or from about 40 nm to about 50 nm. In a furtherembodiment, a substrate-coating transition layer has a thickness lessthan about 10 nm or greater than about 100 nm.

Additionally, in order to reduce problems and disadvantages associatedwith variances in coefficients of thermal expansion betweeninterconnects and electrodes of solid oxide cells, interconnects, insome embodiments of the present invention, comprise compositionalgradients. An interconnect, in one embodiment, comprises a region closerto a cathode and a region further from the cathode, wherein the regioncloser to the cathode has a greater amount of cathode material than theregion of the interconnect further from the cathode. Moreover, inanother embodiment, an interconnect comprises a region closer to ananode and a region further from the anode, wherein the region closer tothe anode has a greater amount of anode material than the region of theinterconnect further from the anode.

In another embodiment, an interconnect comprises a substrate coated witha plurality of metal oxide layers. Layers of the interconnect closer tothe cathode comprise greater amounts of cathode material than layers ofthe interconnect further from the cathode. Moreover, in anotherembodiment, layers of the interconnect closer to the anode comprisegreater amounts of anode material than layers of the interconnectfurther from the anode.

Electrolyzers

Some embodiments of the present invention provide solid oxideelectrolyzer cells or a component thereof comprising a metal oxide. Incertain embodiments, the electrolyzer cell or component thereof issubstantially identical in manufacture and composition as the othersolid oxide cells and components described herein.

In some of those embodiments of the present invention where the samecell can function as an electrolyzer cell and alternately as a fuel cellsimply by reversing the flow of electrons, the cathode of theelectrolyzer corresponds to the fuel electrode of the fuel cell; and theanode of the electrolyzer corresponds to the air electrode of the fuelcell. Those of ordinary skill in the art recognize that oxidation occursat the anode, and reduction occurs at the cathode, so the name of agiven electrode may differ depending on whether the cell is operating asan electrolyzer or as a fuel cell.

In other embodiments, electrons flow in the same direction, regardlessof whether the cell is electrolyzing or producing electricity. This canbe accomplished, for example, by supplying oxygen anions to a givenelectrode in electrolysis mode, and alternately supplying hydrogen tothe same electrode in fuel cell mode. Such an electrode will function asthe oxidizing anode in either mode.

Accordingly, some embodiments of the present invention provide a solidoxide electrolyzer cell, comprising a first electrode, a secondelectrode, and a metal oxide electrolyte interposed between the firstelectrode and the second electrode.

The present invention also provides, in some embodiments, a method formaking a product, comprising:

providing a solid oxide cell comprising a first electrode, a secondelectrode, and a metal oxide electrolyte interposed between the firstelectrode and the second electrode, wherein the metal oxide electrolytehas an ionic conductivity greater than the bulk ionic conductivity ofthe metal oxide;contacting the first electrode with a reactant; andsupplying electrical energy to the first electrode and the secondelectrode thereby causing the reactant to undergo electrochemicalreaction to yield the product.

The skilled electrochemist will appreciate that a complete circuit isnecessary for electrical energy to cause electrochemical reaction. Forexample, at least one ion may traverse the metal oxide electrolyte tocomplete the electrical circuit at the second electrode. Moreover, asecond product may be formed at the second electrode due toelectrochemical reaction. Therefore, some embodiments further providefor contacting the second electrode with a second reactant, therebycausing the second reactant to undergo electrochemical reaction to yielda second product. Contacting an electrode and supplying electricalenergy can occur in any suitable order. In a continuous process,electrical energy supply is maintained while additional reactant(s)enter the cell and product(s) are removed.

Any suitable reactant can be supplied to an electrode forelectrochemical reaction. Suitable reactants include, but are notlimited to, water such as, for example, pure water, fresh water, rainwater, ground water, salt water, purified water, deionized water, watercontaining a ionic substance, brine, acidified water, basified water,hot water, superheated water, steam, carbon dioxide, carbon monoxide,hydrogen, nitrous oxides, sulfur oxides, ammonia, metal salts, moltenmetal salts, and combinations thereof. Ionic substances include thosesubstances that release a ion when placed in contact with water, andinclude, but are not limited to, salts, acids, bases, and buffers.Reactants, and for that matter, products, can be in any suitable form,including solid, liquid, gas, and combinations thereof. Solid reactantsand/or solid products lend themselves to batch processes, althoughsuitable methods for continuously removing a solid product from a cellcan be employed. Fluid reactants and products can appear in either batchor continuous processes. Optionally, heat energy is applied to thereactant, the product, at least one electrode, the metal oxide, thecell, or a combination thereof.

Some embodiments provide a sacrificial electrode. A sacrificialelectrode itself reacts in the electrolysis process, and is therebyconsumed or rendered unreactive as the reaction proceeds. For example, azinc electrode can be consumed in a suitable solid oxide cell reaction,yielding Zn²⁺ and two electrons per atom of zinc consumed. In anotherexample, an electrode can become coated and thereby rendered unreactiveby solid product forming on its surface. The unreactive electrode can beremoved from the cell, and the product extracted from the electrode, orthe product can be used on the electrode in another process. Theelectrode then can be regenerated, recycled, or discarded.Alternatively, a sacrificial electrode can be made to gradually insertinto a cell at a rate consistent with the rate at which the electrode isconsumed.

A reactant undergoing electrochemical reaction can be oxidized and/orreduced, and chemical bonds may form and/or break. For example, whenwater undergoes electrolysis, hydrogen-oxygen bonds break, H⁺ is reducedto H⁰, O²⁻ is oxidized to O⁰, and H₂ and O₂ form, in some circumstances.Hydrogen peroxide and other species may form in other circumstances. Theskilled artisan will appreciate that many electrode half reactions canbe substituted so that any variety of anions, cations, and other speciesmay result from electrochemical reaction.

In one embodiment, water containing NaCl can be electrolyzed to formhydrogen gas and NaOH at the cathode, and chlorine gas at the anode, inthe so-called chlor-alkali process:

2NaCl(aq)+2H₂O(l)→2NaOH(aq)+Cl₂(g)+H₂(g)

A solid oxide cell arranged to carry out that reaction, in someembodiments, provides water containing a high concentration of NaCl (forexample, saturated) to a first electrode that will act as an anode, andprovides water to a second electrode that will act as a cathode. Thecell also provides liquid effluent collection to remove the depletedNaCl solution from the anode, and NaOH-containing water from thecathode. The cell further provides gas effluent collection to removechlorine gas from the anode and hydrogen gas from the cathode.Optionally, the hydrogen and chlorine can be subject to electrochemicalreaction to release the electrochemical energy stored by the foregoingelectrolysis, or they can be used for other industrial processes, suchas the synthesis of sodium hypochlorite.

The present invention also provides methods for storing electrochemicalenergy. In some embodiments, a reactant is supplied to an electrode of asolid oxide cell, the reactant undergoes one or more electrochemicalreactions and yields a fuel, thereby storing electrochemical energy. Theelectrochemical reaction may also yield other products, such as cations,anions, and other species, some of which may form at a second electrodeof the solid oxide cell that completes an electrical circuit. A firstelectrode and a second electrode are separated by a metal oxideelectrolyte in the solid oxide cell. The fuel can be subjected to energyconversion processes such as reverse electrochemical reaction in a fuelcell or battery, combustion, and the like to release the storedelectrochemical energy.

In one embodiment, electrochemical energy is stored by providing areactant to a cathode; reducing the reactant at the cathode to releasean anion and a fuel; storing the fuel; transporting the anion through ametal oxide electrolyte to anode; and oxidizing the anion. Optionally,the oxidized anion is stored as well, separately from the stored fuel.Thus, in one embodiment, water in a suitable form is supplied to acathode, at which it is reduced to hydrogen (H₂) and oxygen anion (O²⁻);the hydrogen is collected and stored, while the oxygen anion diffusesthrough a solid metal oxide electrolyte to an anode where the oxygenanion is oxidized to oxygen (O₂). Optionally, in the foregoingnon-limiting example, the oxygen is collected and stored as well.

When desired, the stored hydrogen can be fed to any suitable fuel cell,including but not limited to the cell that produced the hydrogen, andthe hydrogen can be oxidized to release the stored electrochemicalenergy. Any suitable gas can be fed to the air electrode of the fuelcell, such as, for example, the optionally-stored oxygen, other oxygen,other oxygen-containing gas such as air, and combinations thereof.Alternatively, the stored hydrogen can be combusted with oxygen topropel a rocket, drive a piston, rotate a turbine, and the like. Inother embodiments, the stored hydrogen can be used in other industrialprocesses, such as petroleum cracking.

Some embodiments involve those reactants that yield the high energymaterials commonly found in primary (nonrechargeable) and secondary(rechargeable) batteries. For secondary battery materials, thelow-energy (discharge) state materials may be produced, since secondarybatteries can be charged before first use. Such materials include, butare not limited to, MnO₂, Mn₂O₃, NH₄Cl, HNO₃, LiCl, Li, Zn, ZnO, ZnCI₂,ZnSO₄, HgO, Hg, NiOOH, Ni(OH)₂, Cd, Cd(OH)₂, Cu, CuSO₄, Pb, PbO₂, H₂SO₄,and PbSO₄.

At least some embodiments of fuel cells described above can be used toprovide electrolyzer cell embodiments of the present invention. Whilefuel cell embodiments optionally employ one or more of fuel supply, airor oxidizer supply, interconnects, and electrical energy harvestingmeans (e.g., wires forming a circuit between the fuel and airelectrodes' interconnects), electrolyzer cell embodiments optionallyemploy one or more of reactant supply, fuel collection, interconnects,and electrical energy supply. Optionally, electrolyzer cell embodimentsalso provide collection means for other products in addition to fuel.The reactant supply provides any suitable reactant for electrolysis.Fuel collection, in some embodiments, involves collecting hydrogen forstorage and later use. Storage vessels, metal hydride technology, andother means for storing hydrogen are known in the art. Fuel collection,in other embodiments, involves collection of, for example, carbon-coatedelectrodes for later oxidation. Alternatively, carbon can be formed intofluid hydrocarbon for easy storage and later combustion or reformation.Hydrocarbon formation requires a supply of hydrogen molecules, atoms, orions in a suitable form to combine with carbon at the cathode, in someembodiments. Other product collection involves, in some embodiments, thecollection of oxygen for storage and later use.

In still other embodiments, an electrolyzer cell is capable ofperforming other electrolysis tasks, such as electroplating. In suchembodiments, a metal oxide functions as a solid electrolyte shuttling aion to complete an electrical circuit.

In some embodiments, the electrodes of the electrolyzer cell are adaptedfor the particular electrochemistry expected to occur at the givenelectrode. For example, the electrode can comprise one or more catalyticmaterials to facilitate the electrochemical reaction.

Sensors

Some embodiments of the present invention provide solid oxide sensors orcomponents thereof. Like the fuel cells and electrolyzer cells describedherein, sensors of the present invention comprise a metal oxideelectrolyte. In some embodiments, at least one ion passes through thatmetal oxide electrolyte during cell operation. In other embodiments, thesolid oxide cells useful as sensors or components thereof aresubstantially identical to the solid oxide cells and componentsdescribed above. The metal oxide electrolyte of sensors in certainembodiments has been made according to a process comprising:

applying a metal compound to a substrate, andconverting at least some of the metal compound to a metal oxide,wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the metal oxide. The metal oxideelectrolyte obtains a greater ionic conductivity for example byincluding another material such as a nanobar or a thin sheet asdescribed herein.

Sensors according to various embodiments of the present invention can beused to detect any suitable analyte or analytes. Oxygen sensors, usefulas lambda sensors in automotive exhaust systems, or as oxygen partialpressure detectors in rebreather systems, represent some applicationsfor embodiments. Other sensors, such as gas sensors including but notlimited to CO, CO₂, H₂, NO_(x), and SO_(x) ⁻, ion sensors including butnot limited to pH meters, K⁺, and Na⁺; biosensors including but notlimited to glucose sensors and other enzyme electrodes; electrochemicalbreathalyzers; and electronic noses; represent other applications forembodiments of the present invention.

Many such sensors function at least in part due to the diffusion of anion through an electrolyte, which electrolyte comprises a metal oxide.

Accordingly, additional embodiments provide a method for detecting ananalyte, comprising:

providing a sensor for the analyte, wherein the a sensor comprises ametal oxide made by a process comprising:

-   -   applying a metal compound to a substrate, and    -   converting at least some of the metal compound to the metal        oxide, wherein the metal oxide electrolyte has an ionic        conductivity greater than the bulk ionic conductivity of the        metal oxide; and        passing an ion through the metal oxide to detect the analyte.        The metal oxide electrolyte obtains a greater ionic conductivity        for example by including another material such as a nanobar or a        thin sheet as described herein.

Passing an ion through a metal oxide can include any suitable transportmechanism, such as, for example, diffusion. In addition, movement alongmetal oxide crystal grain boundaries represents another transportmechanism, in some embodiments. Detecting an analyte can indicateobtaining any useful information about the analyte, such as, forexample, determining its mere presence, concentration, partial pressure,oxidation state, or combinations thereof. And, sensors of the presentinvention can be designed for any suitable environment, such as solid,semisolid (e.g., soil), liquid, gas, plasma, and combinations thereof.Also, such sensors can be designed for any suitable operatingtemperature, ranging from the very cold to the very hot. Some solidoxide cells useful as sensors according to the present invention have anoperating temperature of below about −195° C., below about −182° C.,below about −77° C., from about −78° C. to about 0° C., from about 0° C.to about 100° C., from about 100° C. to about 400° C., from about 400°C. to about 600° C., from about 600° C. to about 900° C., from about900° C. to about 1200° C., or above about 1200° C. Other embodimentsuseful as sensors have operating temperatures below about 0° C., aboveabout 0° C., above about 100° C., or above about 500° C.

A few embodiments of the present invention provide solid oxide cells,useful as sensors, that enjoy one or more advantages over conventionalsensors. In some embodiments, the metal oxide has a certain thickness,thinner than conventional sensors. In other embodiments, the solid oxidecell operates at a lower temperature, compared to conventional sensors.Still other embodiments provide smaller sensors. Even other embodimentsprovide sensors made from less-expensive materials. Additionalembodiments have better-matched coefficients of thermal expansionbetween two or more materials in the cell. Still other embodimentsprovide one or more concentration gradients, one or more porositygradients, or combinations thereof.

Further embodiments of the present invention provide a sensor comprisingat least two electrodes separated by a metal oxide that functions as asolid electrolyte. In some of those embodiments, the voltage differencebetween the at least two electrodes corresponds to the concentration ofthe analyte being detected at one of the electrodes. A first electrodefunctions as a reference electrode, and is exposed to a referenceenvironment. Suitable reference environments include, but are notlimited to, air, vacuum, standard solutions, and environments of knownor controlled composition. In some embodiments, the referenceenvironment is formed by arranging one or more materials thatsubstantially isolate the reference electrode from the environment beingmeasured. The second electrode is exposed to the environment beingmeasured. Optionally, the second electrode comprises one or morecatalytic materials. In operation, the first and second electrodes areplaced in electrical communication with one or more devices that canmeasure, for example, the voltage difference, the current, theresistance, or combinations thereof, between the two electrodes. Suchdevices are known in the art. Optionally, heat or cooling can besupplied to one or both electrodes, the electrolyte, or combinationsthereof. Heat or cooling can come from any suitable source, such as, forexample, one or more electrical resistance heaters, chemical reaction,thermal fluid in thermal communication with the sensor, the measuredenvironment, and combinations thereof.

In some embodiments, a reference voltage is supplied to the electrodes,and the current needed to maintain the reference voltage corresponds tothe concentration of the analyte being measured. For example, U.S. Pat.No. 7,235,171, describes two-electrode hydrogen sensors comprisingbarium-cerium oxide electrolyte. The '171 patent also indicates thatvarious other metal oxides also function as electrolytes in hydrogensensors, including selenium cerium oxides, selenium cerium yttriumoxides, and calcium zirconium oxides, which conduct protons, and oxygenanion conductors. The '171 patent is incorporated herein by reference inits entirety.

In other embodiments, a gas permeable porous platinum measuringelectrode is exposed to a measured environment that contains a partialpressure of oxygen. A metal oxide, such as, for example,yttria-stabilized zirconia, separates the measuring electrode from a gaspermeable porous platinum reference electrode that is exposed to air.The voltage difference, current, or both between the electrodes can bemeasured and correlated to the difference of partial pressure of oxygenbetween the measured environment and air. In some embodiments, themeasured environment is an exhaust stream from the combustion ofhydrocarbons.

In still other embodiments, at least two pairs of electrodes appear,wherein a metal oxide separates the electrodes in each pair. One of thetwo pairs functions as a reference cell, while the other of the twopairs functions as a measuring cell, in some embodiments. Furtherembodiments provide, in a first pair of electrodes, a referenceelectrode exposed to a reference environment and a Nernst electrodeexposed to the measured environment. A metal oxide that functions as asolid electrolyte is situated between the reference electrode and theNernst electrode. In a second pair of electrodes, an inner pumpelectrode is separated from an outer pump electrode, with a metal oxidefunctioning as a solid electrolyte situated between the inner and outerpump electrodes. The inner pump electrode and the Nernst electrode areexposed to the environment to be measured optionally through a diffusionbarrier. In operation, an external reference voltage is applied acrossthe pump electrodes. The current needed to maintain the referencevoltage across the pump electrodes provides a measure of the analyteconcentration in the measured environment. For a conventional broadbandlambda sensor containing such a pair of electrodes, see U.S. Pat. No.7,083,710 B2, which is incorporated herein by reference in its entirety.Optionally, a sensor of the present invention is adapted to electricallycommunicate with control circuitry that smoothes operation of the sensorbefore the sensor has achieved standard operating conditions, such astemperature. See, for example, U.S. Pat. No. 7,177,099 B2, which is alsoincorporated herein by reference in its entirety.

Thus, certain embodiments of the present invention provide so-callednarrow band sensors such as lambda sensors that fluctuate between leanand rich indications. Other embodiments provide broadband sensors suchas lambda sensors that indicate the partial pressure of oxygen, andthereby the degree of leanness or richness of an air-fuel mixture.

Some embodiments provide more than two electrodes. For example, a sensoraccording to the present invention may contain a plurality of measuringelectrodes. For another example, a sensor may comprise a plurality ofreference electrodes. In another example, a sensor may comprise, or beadapted to electrically communicate with, a standard electrode or otherdevice providing information useful to the operation of the sensor.

Additional Synthesis and Operational Techniques

Converting a metal compound, according to some embodiments of thepresent invention, comprises exposing the metal compound to anenvironment operable to convert the metal compound to a metal oxide.Environments operable to convert metal compounds to metal oxides, insome embodiments, demonstrate conditions sufficient to vaporize and/ordecompose the compounds and precipitate metal oxide formation. In oneembodiment, an environment operable to convert metal compounds to metaloxides comprises a heated environment. A metal salt of a carboxylicacid, for example, can be exposed to an environment heated to atemperature operable to evaporate the carboxylic acid and induceformation of the metal oxide. In some embodiments, the environment isheated to a temperature greater than about 200° C. In other embodiments,the environment is heated to a temperature ranging from about 400° C. toabout 650° C. In some embodiments, the environment is heated to atemperature of up to about 425° C. or up to about 450° C. In still otherembodiments, the environment is heated to a temperature ranging fromabout 650° C. to about 800° C., or from about 800° C. to about 1000° C.

The time it takes to convert at least a portion of the at least onemetal compound to at least one metal oxide depends on the conversiontechnique. If thermal energy is used to drive the conversion, lowertemperatures generally take a longer time. In some embodiments, a metalcompound composition is heated for at least 15 minutes, at least 30minutes, at least 45 minutes, or at least one hour. In otherembodiments, a metal compound composition is heated for less than 15minutes, or for more than one hour.

In some embodiments, an environment operable to convert metal compoundsto metal oxides, is free or substantially free of oxygen. In otherembodiments, an environment operable to convert metal compounds to metaloxides comprises oxygen.

In some embodiments, the metal compound composition is fully convertedto a metal oxide composition. In some embodiments, the metal compoundcomposition comprises a metal carboxylate, a metal alkoxide, a metalβ-diketonate, or a combination thereof.

In another aspect, the present invention provides methods of increasingthe ionic conductivity of a solid electrolyte. A method of increasingthe ionic conductivity of a solid electrolyte, in some embodiments,comprises increasing the number of grain boundaries in the solidelectrolyte, wherein increasing the number of grain boundaries comprisesforming a plurality of nanocrystalline grains comprising an electrolytematerial. In some embodiments of the present invention, an electrolytematerial comprises one or more metal oxides. Forming a plurality ofmetal oxide nanocrystalline grains comprises applying a metal compoundcomposition to a substrate, and converting at least some of the metalcompound composition to a plurality of metal oxide nanocrystallinegrains.

In another aspect, the present invention provides methods of increasingthe number of triple phase boundaries in a solid oxide cell comprisingproviding an electrolyte comprising a plurality of metal oxidenanocrystalline grains. Providing an electrolyte comprising a pluralityof nanocrystalline grains, in some embodiments, comprises applying ametal compound composition to a substrate, and converting at least someof the metal compound composition to a plurality of metal oxidenanocrystalline grains. In some embodiments, the metal compoundcomposition is fully converted into a plurality of metal oxidenanocrystalline grains. In one embodiment, the substrate comprises anelectrode of a solid oxide cell.

In some embodiments of methods of the present invention, a metalcompound comprises a transition metal compound. In other embodiments, ametal compound comprises a rare earth metal compound. In a furtherembodiment, metal compound compositions comprise a plurality of metalcompounds. In one embodiment, a plurality of metal compounds comprises arare earth metal compound and a transition metal compound. In stillother embodiments, a metal compound comprises metal ions that are thesame or different, and ligands that are the same or different. In someembodiments, those ligands are chosen from one or more carboxylates, oneor more alkoxides, one or more β-diketonates, and combinations thereof.

Moreover, in certain embodiments of the present invention, metalcompound compositions can comprise liquid metal compound compositions,solid metal compound compositions, vapor metal compound compositions, orcombinations thereof. In one embodiment, a liquid metal carboxylatecomposition comprises an excess of the liquid carboxylic acid used toform the metal carboxylate salt. In another embodiment, a liquid metalcompound composition comprises a solvent including, but not limited to,organic solvents such as benzene, toluene, xylene, chloroform,dichloromethane, one or more hydrocarbons such as octane and/or otheralkanes, or mixtures of any of the foregoing. The optional solvent maybe any hydrocarbon and mixtures thereof. In some embodiments, thesolvent can be chosen from carboxylic acids; toluene; benzene; xylene;alkanes, such as for example, propane, butane, isobutene, hexane,heptane, octane, and decane; alcohols, such as methanol, ethanol,n-propanol, isopropanol, n-butanol, and isobutanol; mineral spirits;β-diketones, such as acetylacetone; ketones such as acetone;high-paraffin, aromatic hydrocarbons; and combinations of two or more ofthe foregoing. Some embodiments employ solvents that contain no water orwater in trace amounts or greater, while other embodiments employ wateras the solvent. In some embodiments, the metal compound compositionfurther comprises at least one carboxylic acid. Some embodiments employno solvent in the metal compound composition. Other embodiments employno carboxylic acid in the metal compound composition. In someembodiments, solid metal compound compositions comprise metal compoundpowders. In a further embodiment, a vapor metal compound compositioncomprises a gas phase metal compound operable to condense on a substrateprior to conversion to a metal oxide. In one embodiment, a metalcompound composition comprises a gel including, but not limited to, asol-gel, hydrogel, or a combination thereof.

Establishing a porosity gradient among a plurality of layers of anelectrode permits the electrode to better match the porosity of theelectrolyte without producing a pore structure within the electrode thatis unduly restrictive to air, fuel, reactant, or product flow. Porositycan be controlled by any suitable method, such as, for example, byincluding particles such as nanoparticles in the compositions used tomanufacture an electrode or electrolyte, pore-forming agents thatrelease gas during manufacture, substances that can be dissolved,melted, or sublimed and thereby removed after a given layer has beenmanufactured, and combinations thereof.

In some embodiments, a solid oxide cell has an operating temperatureless than about 1000° C. or less than about 900° C. In anotherembodiment, a solid oxide fuel cell of the present invention has anoperating temperature of less than about 800° C., less than about 700°C., less than about 600° C., or less than about 500° C. In a furtherembodiment, a solid oxide fuel cell of the present invention has anoperating temperature of less than about 300° C., less than about 200°C., or less than about 100° C.

A lower operating temperature allows non-ceramic materials such asmetals and metal carbides to be used. Since these materials generallypossess higher levels of mechanical or structural strength at the loweroperating temperatures, they can have higher levels of porosity thaneither ceramics (such as the LSM that can be used for cathodes in fuelcells) or cermets (such as mixtures of nickel and zirconia that can beused for anodes in fuel cells). In other embodiments, solid oxide cellsof the present invention demonstrate greater tolerance for highoperating temperatures. That greater tolerance enables such cells to beconstructed from less expensive materials, and may increase servicelifetime. The increased tolerance for high operating temperatures stemsfrom the greater matching of coefficients of thermal expansion availableto at least some embodiments of the present invention.

In yet another aspect, the present invention provides a method ofgenerating electric current comprising providing a solid oxide fuel cellcomprising an air electrode, a fuel electrode, an electrolyte interposedbetween the air electrode and the fuel electrode wherein the electrolytecomprises a metal oxide and another material and has an ionicconductivity greater than the bulk ionic conductivity of the metal oxideand the other material, and optionally an electrode-electrolytetransition layer; providing a fuel to the fuel electrode; providingoxygen to the air electrode; oxidizing the fuel to generate freeelectrons; transporting the free electrons through an external circuitto the air electrode (cathode); and then reducing the diatomic oxygenmolecules at the air electrode to oxygen anions. In some embodiments,the fuel comprises hydrogen. In other embodiments, the fuel comprises ahydrocarbon. In embodiments wherein the fuel is a hydrocarbon, methodsof generating electrical current further comprise reforming thehydrocarbon fuel at the fuel electrode. Other embodiments provide cellscomprising more than one cathode electrode, and/or more than one anodeelectrode. Still other embodiments provide a plurality of cells, whereinthe cells are connected in series, in parallel, or a combinationthereof.

FIG. 2 illustrates a solid oxide fuel cell according to one embodimentof the present invention. As displayed in FIG. 2, the solid oxide fuelcell comprises an air electrode (cathode), a fuel electrode (anode), anelectrolyte interposed between the air electrode (cathode) and the fuelelectrode (anode). An electrode-electrolyte transition layer (not shown)is optionally interposed between the air electrode (cathode) and theelectrolyte. The air electrode (cathode) and the fuel electrode (anode)are connected by an external circuit across which a load is applied.Oxygen (O₂) or a mixture of gases comprising oxygen (e.g., air) is fedto the air electrode wherein oxygen molecules are reduced to oxygenanions (O²⁻). Moreover, hydrogen molecules (H₂) from a fuel source areoxidized at the fuel electrode. Electrons removed from hydrogenmolecules at the fuel electrode travel through interconnects (not shown)to the external circuit to the air electrode (cathode) generatingelectric current while oxygen anions (O²⁻) travel through theelectrolyte to combine with hydrogen cations (H⁺) thereby producingwater (H₂O).

In other embodiments of the present invention, various configurations offuel cells are contemplated. For example, more than one fuel electrodecan pair with more than one air electrode. The physical configuration ofthe various electrodes, electrolytes, interconnects, and/or othercomponents is not limited. In some embodiments, the configuration isoptimized for size, current density, voltage, portability, fuelversatility, energy conservation, specific application, aesthetics,other considerations, or combinations thereof.

EXAMPLES Example 1

FIG. 3 depicts one embodiment of the invention in the form of a solidoxide cell having a metal oxide electrolyte 380 positioned between afirst electrode 310 and a second electrode 320. The metal oxideelectrolyte 380 comprises a powder 350 together with a metal oxide 360.In some cases, the powder 350 can be mixed with one or more metalcompounds to form a slurry that is then applied by spin coating,brushing, or other suitable method onto the first electrode 310 (orsecond electrode 320). Then, the metal compound is converted to form themetal oxide 360, for example, by heating the atmosphere about the metalcompound, or by inductively heating the first electrode 310. Optionally,once a layer of the metal oxide electrolyte 380 has been formed,additional powder-metal compound slurry can be applied and heated toform a thicker metal oxide electrolyte 380. The cell is assembled byplacing the second electrode 320 onto the metal oxide electrolyte 380,or, optionally, additional metal compound (or powder-metal compoundslurry) is converted to metal oxide while contacting the metal oxideelectrolyte 380 and the second electrode 320, to provide better contactbetween the metal oxide electrolyte 380 and the second electrode 320. Incertain cases, the powder 350 is strontium titanate, and the metal oxide360 is yttria-stabilized zirconia.

In operation, for example, air or other oxygen-containing gas issupplied to the first electrode 310, which acts as the cathode to reducediatomic oxygen to O²⁻. O²⁻ (shown as O⁼) then diffuses through themetal oxide electrolyte 380 to the second electrode 320, where the O²⁻joints H⁺ to form water (not shown). The H⁺ results from the oxidationof, for example, hydrogen gas at the second electrode 320, which acts asan anode. Circuitry (not shown) transmits electrons from the anode(second electrode 320) to the cathode (first electrode 310).

Example 2

FIG. 4 depicts another embodiment of the present invention, in which afirst metal oxide 450 and a second metal oxide 460, disposed ininterpenetrating domains of metal oxides, form a metal oxide electrolytebetween two electrodes 410, 420. To form such domains, a first metalcompound composition is applied to the first electrode 410 and convertedto a first metal oxide 450, such as, for example, strontium titanate.Then, a second metal compound composition is applied to the first metaloxide 450 and allowed to accumulate in pores, imperfections, and defectsin the first metal oxide so formed. The second metal oxide compositionis converted to form a second metal oxide 460, such as, for example,yttria-stabilized zirconia. Six alternating layers of the first metaloxide 450 and the second metal oxide 460 are formed in this embodiment.

In operation, for example, air or other oxygen-containing gas issupplied to the first electrode 410, which acts as the cathode to reducediatomic oxygen to O²⁻. O²⁻ (shown as O⁼) then diffuses through themetal oxide electrolyte 480 to the second electrode 420, where the O²⁻joints H⁺ to form water (not shown). The H⁺ results from the oxidationof, for example, hydrogen gas at the second electrode 420, which acts asan anode. Circuitry (not shown) transmits electrons from the anode(second electrode 420) to the cathode (first electrode 410).

Example 3

FIG. 5 depicts a solid oxide cell according to one embodiment of thepresent invention. A nanobar 540 and a metal oxide 560, disposed so thatthe nanobars 540 orient substantially perpendicularly to a first planarelectrode 510, form a metal oxide electrolyte 580 between two electrodes510, 520. The nanobar 540 can be, for example, a multi-walled carbonnanotube of semiconductor characteristic, oriented in metal oxide 560which can be, for example, yttria-stabilized zirconia. To make the cellof FIG. 5, chosen nanobars 540 are combined with at least one metalcompound in a metal compound composition, that is then applied to thefirst electrode 510. An orienting force is then applied. Optionally, thefirst electrode with the metal compound composition is placed in amagnetic field, at least a portion of the nanobars orient due to themagnetic field, and the metal compound composition is converted to formthe metal oxide 560. Or, an electric field is applied to orient at leasta portion of the nanobars 540, and the metal compound composition isconverted to form the metal oxide 560. In some cases, the secondelectrode 520 is placed over the metal compound composition on the firstelectrode 510, and an electric field is established between the firstelectrode 510 and the second electrode 520, thereby orienting at least aportion of the nanobars 540. Then the metal compound composition isconverted, such as, for example, by heating, thereby forming the metaloxide 560 and the metal oxide electrolyte 580.

In operation, for example, air or other oxygen-containing gas issupplied to the first electrode 510, which acts as the cathode to reducediatomic oxygen to O²⁻. O²⁻ (shown as O⁼) then diffuses through themetal oxide electrolyte 580 to the second electrode 520, where the O²⁻joints H⁺ to form water (not shown). The H⁺ results from the oxidationof, for example, hydrogen gas at the second electrode 520, which acts asan anode. Circuitry (not shown) transmits electrons from the anode(second electrode 520) to the cathode (first electrode 510).

Example 4

FIG. 6A depicts thin sheets 650 interspersed with metal oxide 660. Toassemble the thin sheets 650 with metal oxide 660, a metal compoundcomposition is applied to a first thin sheet 650, and a second thinsheet 650 is laid over the metal compound composition. Then, the metalcompound composition is converted to a metal oxide. Additional metalcompound composition (which in other embodiments may be different fromthe metal compound composition applied to the first thin sheet 650) isapplied to the exposed surface of the second thin sheet 650, and a thirdthin sheet 650 is laid over the metal compound composition. That metalcompound composition is then converted to form the metal oxide 660.Accordingly, additional thin sheets 650 and additional metal oxide 660are assembled, in this embodiment. Alternatively, multiple thin sheets650 can be coated with metal compound composition, held together under amild compressive force, and heated to convert the metal compound intometal oxide 660. In still another variant, each thin sheet 650 can havemetal compound applied on both sides, and the metal compound is thenconverted into metal oxide 660.

Thin sheets 650 thus coated on both sides with metal oxide 660 can thenbe assembled together. Optionally, one or more epoxies (not shown) canassist to hold the thin sheets 650 and metal oxide 660 together.

The thin sheets 650 and metal oxide 660 of FIG. 6A can be sliced alongplane “A” and assembled into cells of the present invention. FIG. 6Bshows the assembly of thin sheets 650 and metal oxide 660 depicted inFIG. 6A as if cut along “A.” Two planar electrodes sandwiching theassembly of FIG. 6B form a cell in one embodiment of the presentinvention. In operation, ions would diffuse substantially parallel tothe thin sheets 650. In some embodiments, the thin sheets 650 are mica,and the metal oxide 660 is yttria-stabilized zirconia.

Example 5

FIG. 7A depicts thin sheets 750 such as mica formed into flat annulardiscs, such as by cutting and pressing hot mica, and arranged in spaceso the discs are substantially parallel. As in Example 4, the thinsheets 750 can be coated with metal oxide (not shown) in any suitablemanner and sequence.

The thin sheets 750 of FIG. 7A (together with metal oxide, not shown)can be assembled between an outer tubular electrode 710 and an innertubular electrode 720 to form a metal oxide electrolyte 780 shown inFIG. 7B. The outer tubular electrode 710 and inner tubular electrode 720shown are circular in cross-section; in other embodiments, tubes havingany suitable cross-section may be used. In operation, for example, as asolid oxide fuel cell, fuel such as hydrogen-containing gas isintroduced in cavity 790 where it is oxidized at inner tubular electrode720, which acts as an anode. Oxygen-containing gas such as air contactsthe outer tubular electrode 710, which acts as a cathode reducing oxygento O²⁻. The oxygen anions migrate from the outer tubular electrode 710through the metal oxide electrolyte 780 to the inner tubular electrode720, where the oxygen anions combine with protons to form water, whichflows out of the cell through cavity 790. External circuitry (not shown)completes the circuit between the outer tubular electrode 710 and theinner tubular electrode 720. Optionally, outer tubular electrode 710and/or inner tubular electrode 720 are porous. In another example, oneor more epoxies (not shown) can help seal and/or hold together the metaloxide electrolyte 780.

Example 6

FIG. 8 depicts a side cut-away view of a solid oxide cell according toan embodiment of the present invention, optionally operable to test ametal oxide electrolyte 880 for enhanced ionic conductivity. A cathode810 and an anode 820 sandwich a metal oxide electrolyte 880 to testperformance with external circuitry 870. An inner tube 804, for exampleglass, supports the cell by way of an annular seal 830, which comprisesone or more epoxies. The inner tube 804 supplies a hydrogen-containinggas (H2) to the second electrode 820, which acts as an anode. Firstelectrode 810 acts as a cathode, reducing oxygen in an oxygen-containinggas (AIR) to O²⁻, which then migrates through metal oxide electrolyte880 to the second electrode 820. Outer tube 802 contains the cell andoptionally allows control over the oxygen content and the temperature ofthe cell. External circuitry 870 creates a circuit from first electrode810 to second electrode 820, and allows determination of oxygen anionconductivity of metal oxide electrolyte 880 using an ampmeter (A) and avoltimeter (V).

EMBODIMENTS Embodiment 1

A method of enhancing ionic conductivity in a metal oxide electrolytecomprising a first material and a metal oxide comprising:

applying a metal compound to the first material; andconverting at least some of the metal compound to form the metal oxide;wherein the first material and the metal oxide have an ionicconductivity greater than the bulk ionic conductivity of the firstmaterial and of the metal oxide.

Embodiment 2

The method of embodiment 1, wherein the first material comprisescrystalline material.

Embodiment 3

The method of embodiment 2, wherein the crystalline material comprisesnanocrystalline material.

Embodiment 4

The method of embodiment 1, wherein the first material comprises a metaloxide.

Embodiment 5

The method of embodiment 1, wherein the first material is chosen fromstrontium titanate, titania, alumina, zirconia, yttria-stabilizedzirconia, alumina-doped yttria-stabilized zirconia, iron-doped zirconia,magnesia, ceria, samarium-doped ceria, gadolinium-doped ceria, andcombinations thereof.

Embodiment 6

The method of embodiment 5, wherein the first material is chosen fromalumina, titania, zirconia, yttria-stabilized zirconia, alumina-dopedyttria-stabilized zirconia, iron-doped zirconia, magnesia, ceria,samarium-doped ceria, gadolinium-doped ceria, and combinations thereof.

Embodiment 7

The method of embodiment 1, wherein the first material comprises mica.

Embodiment 8

The method of embodiment 1, wherein the metal oxide is chosen fromstrontium titanate, titania, alumina, zirconia, yttria-stabilizedzirconia, alumina-doped yttria-stabilized zirconia, iron-doped zirconia,magnesia, ceria, samarium-doped ceria, gadolinium-doped ceria, andcombinations thereof.

Embodiment 9

The method of embodiment 8, wherein the metal oxide is chosen fromalumina, titania, zirconia, yttria-stabilized zirconia, alumina-dopedyttria-stabilized zirconia, iron-doped zirconia, magnesia, ceria,samarium-doped ceria, gadolinium-doped ceria, and combinations thereof.

Embodiment 10

The method of embodiment 1, wherein the first material comprisesstrontium titanate, and the metal oxide comprises yttria-stabilizedzirconia.

Embodiment 11

The method of embodiment 10, wherein the yttria-stabilized zirconiacomprises from about 10 mol % to about 20 mol % yttria.

Embodiment 12

The method of embodiment 10, wherein the yttria-stabilized zirconiacomprises from about 12 mol % to about 18 mol % yttria.

Embodiment 13

The method of embodiment 10, wherein the yttria-stabilized zirconiacomprises from about 14 mol % to about 16 mol % yttria.

Embodiment 14

The method of embodiment 1, wherein the first material comprisesmagnesia, and the metal oxide comprises yttria-stabilized zirconia.

Embodiment 15

The method of embodiment 1, wherein the first material comprisestitania, and the metal oxide comprises yttria-stabilized zirconia.

Embodiment 16

The method of embodiment 1, wherein the first material comprisesstrontium titanate, and the metal oxide comprises iron-doped zirconia.

Embodiment 17

The method of embodiment 1, wherein the first material comprisessamarium-doped ceria, and the metal oxide comprises ceria.

Embodiment 18

The method of embodiment 1, further comprising:

applying an epoxy to the metal oxide.

Embodiment 19

A metal oxide electrolyte comprising:

a first material and a metal oxide, wherein the metal oxide is formed byapplying a metal compound to the first material; andconverting at least some of the metal compound to form the metal oxide,wherein the first material and the metal oxide have an ionicconductivity greater than the bulk ionic conductivity of the firstmaterial and of the metal oxide.

Embodiment 20

A method for forming a metal oxide electrolyte, comprising:

applying a metal compound to a first material in powder form; andconverting at least some of the metal compound to form a metal oxide,thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first material and of the metaloxide.

Embodiment 21

The method of embodiment 20, wherein the first material in powder formcomprises strontium titanate, and the metal oxide comprisesyttria-stabilized zirconia.

Embodiment 22

The method of embodiment 20, wherein the first material in powder formcomprises mica, and the metal oxide comprises yttria-stabilizedzirconia, gadolinium-doped ceria, alumina, or a combination thereof.

Embodiment 23

The method of embodiment 20, further comprising applying an orientingforce before, during, or before and during the converting.

Embodiment 24

The method of embodiment 23, wherein the orienting force is chosen frommagnetic fields, electric fields, and combinations thereof.

Embodiment 25

A method for forming a metal oxide electrolyte, comprising:

applying a first metal compound to a substrate;converting at least some of the first metal compound to form a firstmetal oxide on the substrate;applying a second metal compound to the substrate comprising the firstmetal oxide; andconverting at least some of the second metal compound to form a secondmetal oxide on the substrate comprising the first metal oxide, therebyforming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first metal oxide and of thesecond metal oxide.

Embodiment 26

The method of embodiment 25, further comprising

applying additional first metal compound to the substrate comprising thefirst metal oxide and the second metal oxide; andconverting at least some of the additional first metal compound to formadditional first metal oxide.

Embodiment 27

The method of embodiment 26, further comprising

applying additional second metal compound to the additional first metaloxide; andconverting at least some of the additional second metal compound to formadditional second metal oxide.

Embodiment 28

A method for forming a metal oxide electrolyte, comprising:

applying a metal compound to a first material in nanobar form; andconverting at least some of the metal compound to form a metal oxide,thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the first material and of the metaloxide.

Embodiment 29

The method of embodiment 28, wherein the nanobar form is chosen fromnanorods, single-walled nanotubes, multiwalled nanotubes, andcombinations thereof.

Embodiment 30

The method of embodiment 28, wherein the first material in nanobar formis present in the metal oxide electrolyte conforming to an orientation.

Embodiment 31

The method of embodiment 30, wherein the orientation is caused by amagnetic field applied before, during, or before and during theconverting.

Embodiment 32

The method of embodiment 31, wherein the magnetic field is chosen fromstatic magnetic fields, variable magnetic fields, uniform magneticfields, non-uniform magnetic fields, and combinations thereof.

Embodiment 33

The method of embodiment 30, wherein the orientation is caused by anelectric field applied before, during, or before and during theconverting.

Embodiment 34

The method of embodiment 28, wherein the first material in nanobar formcomprises strontium titanate, and the metal oxide comprisesyttria-stabilized zirconia.

Embodiment 35

A method for forming a metal oxide electrolyte comprising:

applying a metal compound to a thin sheet; andconverting at least some of the metal compound to form a metal oxide onthe thin sheet, thereby forming the metal oxide electrolyte;wherein the metal oxide electrolyte has an ionic conductivity greaterthan the bulk ionic conductivity of the thin sheet and of the metaloxide.

Embodiment 36

The method of embodiment 35, further comprising applying an epoxy to themetal oxide electrolyte.

Embodiment 37

The method of embodiment 35, further comprising applying an epoxy to themetal oxide.

Embodiment 38

A method for making a metal oxide electrolyte, comprising:

applying a nanobar functionalized with a metal compound to a substrate;andconverting the metal compound to a metal oxide, thereby forming themetal oxide electrolyte;

-   -   wherein the metal oxide electrolyte has an ionic conductivity        greater than the bulk ionic conductivity of the metal oxide.

Embodiment 39

The method of embodiment 38, wherein the nanobar functionalized with ametal compound is oriented before the converting.

Embodiment 40

The method of embodiment 39, wherein the nanobar functionalized with ametal compound is oriented by the applying.

Embodiment 41

The method of embodiment 39, wherein the nanobar functionalized with ametal compound is oriented by brushing, spin coating, a magnetic field,an electric field, or a combination thereof.

Embodiment 42

A solid oxide cell, comprising:

an inner tubular electrode having an outer surface;an outer electrode; anda metal oxide electrolyte adapted to provide ionic conductivity betweenthe inner tubular electrode and the outer electrode;

-   wherein the metal oxide electrolyte comprises a plurality of thin    sheets oriented substantially perpendicular to the outer surface of    the inner tubular electrode, and a metal oxide contacting the thin    sheets.

Various embodiments of the invention have been described in fulfillmentof the various objects of the invention. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Furthermore, the foregoing description of variousembodiments does not necessarily imply exclusion. For example, “some”embodiments may include all or part of “other” and “further”embodiments. Numerous modifications and adaptations thereof will bereadily apparent to those skilled in the art without departing from thespirit and scope of the invention. As used throughout this document andin the claims, “a” does not necessarily mean “one and only one.” Unlessotherwise indicated, “a” can mean “at least one.” For example, “a metaloxide electrolyte comprising a first material and a metal oxide”indicates a metal oxide electrolyte that comprises one or more materials(which may include a metal oxide), and one or more metal oxides.

We claim:
 1. A method for forming a metal oxide electrolyte, comprising:applying a metal compound to a first material in nanobar form; andconverting at least some of the metal compound to form a metal oxide,thereby forming the metal oxide electrolyte; wherein the metal oxideelectrolyte has an ionic conductivity greater than the bulk ionicconductivity of the first material and of the metal oxide; wherein thefirst material in nanobar form is present in the metal oxide electrolyteconforming to an orientation, and the orientation is caused by amagnetic field applied before, during, or before and during theconverting.
 2. The method of claim 1, wherein the nanobar form is chosenfrom nanorods, single-walled nanotubes, multiwalled nanotubes, andcombinations thereof.
 3. The method of claim 1, wherein the magneticfield is chosen from static magnetic fields, variable magnetic fields,uniform magnetic fields, non-uniform magnetic fields, and combinationsthereof.
 4. The method of claim 1, wherein the first material in nanobarform comprises strontium titanate, and the metal oxide comprisesyttria-stabilized zirconia.
 5. A method for forming a metal oxideelectrolyte, comprising: applying a metal compound to a first materialin nanobar form; and converting at least some of the metal compound toform a metal oxide, thereby forming the metal oxide electrolyte; whereinthe metal oxide electrolyte has an ionic conductivity greater than thebulk ionic conductivity of the first material and of the metal oxide;wherein the first material in nanobar form is present in the metal oxideelectrolyte conforming to an orientation, and the orientation is causedby an electric field applied before, during, or before and during theconverting.
 6. The method of claim 5, wherein the nanobar form is chosenfrom nanorods, single-walled nanotubes, multiwalled nanotubes, andcombinations thereof.
 7. The method of claim 5, wherein the electricfield is chosen from static electric fields, variable electric fields,uniform electric fields, non-uniform electric fields, and combinationsthereof.
 8. The method of claim 5, wherein the first material in nanobarform comprises strontium titanate, and the metal oxide comprisesyttria-stabilized zirconia.